WATER RESOURCES STUDY
PURCELLVILLE, VIRGINIA
MARCH 14, 2000
ENGINEERING CONCEPTS, INC. FINCASTLE, VIRGINIA 24090 540-473-1253
with HSI GEOTRANS GLEN ALLEN, VIRGINIA 23060 804-346-5433
TABLE OF CONTENTS
Executive Summary ...... 4 I. Introduction...... 6 A. Scope of Study...... 9 B. Target Water Quantities...... 10 II. Neighboring System Review ...... 12 III. Initial Source Review...... 14 A. Wells...... 14 B. Reservoirs...... 15 C. River Connections...... 17 IV. Detailed Source Review ...... 23 V. Wells...... 24 A. Introduction...... 24 B. Groundwater Conditions ...... 25 C. Groundwater Availability ...... 40 D. Recommended Groundwater Exploration Areas...... 41 E. Costs of Development...... 43 F. Conclusions ...... 45 G. Recommendations...... 46 VI. Mountain Resource ...... 48 A. Sources...... 48 B. Water Quantity...... 49 C. Capturing the Resource...... 52 D. Other Issues...... 54 E. Individual Sources...... 54 F. Potential Resource Development Locations ...... 55 G. Yield...... 59 H. Pipeline from Cooper Spring...... 61 I. Costs of Development...... 62 J. Recommendations...... 63 VII. Sleeter Lake ...... 64 VIII. Summary...... 67 IX. Conclusions...... 70 X. Recommendations...... 72
Appendices Figures & Maps ...... Appendix A Geologic Formations ...... Appendix B Spring Flow Data...... Appendix C Sleeter Lake Test Results ...... Appendix D Contact List...... Appendix E References...... Appendix F
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Executive Summary
On May 24, 1999 the Town of Purcellville, Virginia directed Engineering Concepts, Inc. (ECI) of Fincastle, Virginia to perform a Water Resources Study of alternatives for future municipal water supplies for the Town.
ECI initially reviewed 12 proposed options and reported the advantages and disadvantages of each. This information was conveyed to the Town in a report on August 9, 1999. The text of that report is included in this document.
On August 31, 1999, after review of this initial report, the Town directed ECI to make a more detailed investigation of six of these options. These were:
1) New wells 2) The Mountain Resource, including: a) Cooper Reservoir b) New Springs c) Expanding Hirst Reservoir d) Piping water from Cooper Spring to Hirst Reservoir 3) Sleeter Lake
In considering the above alternatives, nine variations were investigated. These were:
1) New wells 2) The Mountain Resource, including: a) Reservoirs of different heights at five locations b) Piping water from Cooper Spring to Hirst Reservoir 3) Sleeter Lake
Upon review of previously developed demand forecasts and looking at current demand trends, the 2050 demand for the Town of Purcellville water system was set at 2.0 million gallons per day (MGD).
After analyzing the ability of these alternatives to meet the forecast demand, ECI has determined that none of these alternatives will likely be able to meet 2050 water demands for the Town of Purcellville on its own. Consequently, ECI recommend that the Town concurrently pursue three of these alternatives. These are:
• New Wells • A new reservoir off Spring Crossing Road, at a site designated in the report as Site E. • Use of Sleeter Lake as a municipal water source.
These recommendations are not in any priority order, as all three options should be pursued. In addition to direct pursuit of these water supply alternatives, ECI has also recommended a daily data collection procedure, and preliminary engineering studies of
\Water Resource Study.doc 4 3/14/00 the recommended alternatives. These activities will support and refine the eventual balance of water resources that will be supplied by each component of the eventual water supply network.
Components:
New Wells. The following steps are recommended in pursuit of new wells:
1. Conduct detailed geologic mapping. 2. Identify specific properties that are favorable for developing groundwater resources. 3. Conduct surface geophysical surveys. 4. Prepare specifications and solicit bids from qualified well drillers. 5. Develop a source water protection plan for each new groundwater source. 6. Develop a groundwater level and streamflow-monitoring program to monitor the long-term affects of the groundwater withdrawal.
Site E . The following steps are recommended in pursuit of an impoundment at Site E.
1. Initiate contact with the property owner(s). 2. Implement daily data collection program. 3. Perform preliminary engineering of the dam site.
Sleeter Lake . The following steps are recommended in pursuit of Sleeter Lake.
1. Initiate contact with the property owner. 2. Conduct a through water quality test of lake water samples. 3. Perform preliminary engineering on water treatment processes and pipeline routes.
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I. Introduction
The Town of Purcellville, located in western Loudoun County, operates a municipal water supply system to serve residents in Town and portions of its designated Urban Growth Area (UGA). The Town currently uses three wells and three mountain springs as its water sources. Flows from the three springs are routed to the J.T. Hirst Reservoir located at the foot of the eastern slope of the Blue Ridge. See Map 1 in Appendix A.
Together these sources are supplying an average demand of 0.34 million gallons per day (MGD). Figure 1-1 below shows the growth of usage from August 21, 1995 through August 8, 1999. Figure A-1 in Appendix A shows daily usage from the three springs as measured at the Town’s Water Treatment Plant. Usage from the reservoir has grown steadily over the past four years to a current average of about 220,000 gallons per day. Figure A-2 in Appendix A shows the ratio of groundwater to surface water usage over the same period. As can be seen, the trendline shows a minor shift away from surface water and toward a slightly larger share of groundwater. Overall, the ratio has remained around 65% surface water and 35% groundwater.
Figure 1-1 Daily Water Usage with Trendline All Sources Town of Purcellville, Virginia
800,000
700,000
600,000
500,000
400,000
Gallonsper Day 300,000
200,000
100,000
0 1/1/95 1/1/96 1/1/97 1/1/98 1/1/99 Date
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Previously published figures have indicated the safe yield from the Hirst system to be 0.24 or 0.30 MGD. Values calculated for this report do not disagree with these figures.
The three wells currently in use are designated as Main Street # 1, Main Street # 5, and Cornwell. The two Main Street wells are located in the Main Street Village townhouse community, located at the east end of Town. The Cornwell Well is located west of Hillsboro Road in the northwest portion of Town.
The three springs are located approximately six miles northwest of Town on the east slope of the Blue Ridge. These springs are Harris Spring, Potts Spring and Cooper Spring.
Springs and Reservoir
Harris and Potts Springs are located approximately 250 feet from each other and approximately 1,000 feet upslope of the Town’s J. T. Hirst Reservoir. The reservoir is located at the end of Potts Lane off Edgegrove Road (Route 716). The reservoir consists of three separate but interconnected pools. A dam that is approximately 400 feet long and 30 feet high forms the largest of the three pools. The pool impounded by this dam has a surface area of approximately nine acres. Its normal water surface elevation is about 742 feet.
Upstream of this pool, and located to the left (facing downstream), is a much smaller pool with approximately the same water surface elevation. An embankment that is used as an access road separates this second pool from the main one. The second pool has a surface area of approximately 0.2 acre.
Upstream of the second pool and behind the main pool is a third impoundment. It is separated from the first two pools by another embankment. The water surface in the third pool is approximately 10 feet higher than that in the first two pools and covers an area of about 3.5 acres.
Individual volumes of each pool have not been noted, but Dewberry & Davis has reported a total volume of 39 million gallons (120 acre feet).
The third spring, Cooper Spring, is located approximately 1.75 miles north of Harris and Potts Springs. It is at an elevation of 885 feet on the east slope of the Blue Ridge. A small pool at the spring, about three feet deep, impounds water to be transmitted to the Town’s water treatment plant.
Records collected approximately once per week from the flow meter in the pipeline from Cooper indicate that currently an average of 80,000 gallons of water per day are being used from Cooper Spring. A chart of available data is presented as Figure A-3 in Appendix A.
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Conveyance from Springs to Water Treatment Plant
The Town’s water treatment plant (WTP) is located on Short Hill Road (Rt 716) about 3.6 miles from the center of Town. The plant treats surface waters from the three springs. Water is conveyed approximately 2.5 miles to the plant from Hirst Reservoir via parallel 6” and 12” pipelines.
Water from Cooper Spring is conveyed approximately 1.5 miles from a small collecting pool at the spring via a 6” pipeline. This pipeline runs from the spring to a connection with the Hirst to WTP pipelines. The exact piping configuration at the connection point is unclear due to modifications to the system over the years. Based on the most reliable information the configuration is probably similar to that shown in Figure 1-2 below.
Figure 1-2 Assumed Piping Configuration Below Hirst Dam
6“ line from Cooper Spring
Hirst Dam 6” line to WTP 12” line to WTP
possible line interconnection pipe
Flow from the sources to the water treatment plant is by gravity. The approximate elevations of these various components are listed here:
Table 1-1 Elevations of Key Sites
Site Approximate Elevation
Cooper Spring 885 Harris Spring 825 Potts Spring 825 J.T. Hirst Reservoir upper pool 751 J.T. Hirst Reservoir lower pools 741 Water Treatment Plant 710
The relative proportions of water that come from the Hirst Reservoir and the Cooper Spring sources are a function of the hydraulics of the pipeline system. These can be
\Water Resource Study.doc 8 3/14/00 estimated based on assumed conditions. However, actual field conditions could vary from the assumptions presented here.
A hydraulic model of the plant supply pipe network was developed to determine how this system performs. For modeling purposes, pipeline conditions are assumed to be relatively consistent through the piping network between Cooper Spring, Hirst Reservoir and the WTP. No partially closed valves or other impediments to flow along the lines were assumed. Based on the elevations presented above, the model indicates that due to the much higher elevation of Cooper Spring the plant will draw water first from this source, up to the capacity of the spring. Therefore, if the treatment plant were operating at 350 gallons per minute (gpm) and sufficient water was available from Cooper Spring, all 350 gpm would come from that source. It is only because Cooper Spring cannot meet the demand, which happens at a level well below 350 gpm, that water is drawn from Hirst Reservoir. Since Cooper typically supplies much less than 350 gpm, this implies that Cooper Spring is possibly being used almost to its full extent.
Existing Wells
The Town currently uses three existing wells, all of which are located in Town.
The Cornwell Well is located off Hillsboro Road. It is 271 feet deep and generally produces about 57 gallons per minute. The pump is set at a depth of 170 feet, just below the major water producing zone.
Two additional wells are located in the Main Street Village, south of East Main Street. Well # 1 is 565 feet deep and well # 5 is 525 feet deep. Together these wells are producing about 159 gallons per minute.
A third unused well is also located in Main Street Village. Well # 2 was believed to have had a capacity of about 155 gallons per minute, however the yields of the other two wells have been downgraded since these tests were performed.
A. Scope of Study
On May 24, 1999, the Town of Purcellville contracted with Engineering Concepts, Inc. (ECI) of Fincastle, Virginia to perform a Water Resources Study to assess the availability of new water sources for the Town. HSI Geotrans of Glen Allen, Virginia is assisting ECI with the groundwater portion of the evaluation. An initial review of all proposed options was delivered to the Town on August 9, 1999, and is included in this report.
On August 31, 1999 the Town of Purcellville directed Engineering Concepts to investigate three options for water resources availability. These options were:
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1) New wells 2) The Mountain Resource, including: c) Cooper Reservoir d) New Springs e) Expanding Hirst Reservoir f) Piping water from Cooper Spring to Hirst Reservoir 3) Sleeter Lake
Subsequent direction from the Town indicated that efforts should concentrate on the groundwater (i.e. new wells) and Mountain Resource options. Investigations conducted relative to the Sleeter Lake option are included herein, but the level of detail is less than that presented for the other two options
B. Target Water Quantities
1. The 1996 Water and Wastewater Master Plan cited target water quantities of 2.0 million gallons per day (MGD) for a ‘Town only’ water system and 4.0-MGD for a Western Loudoun County regional system.
2. ECI reviewed the projected demand for a ‘Town only’ system and determined that in light of the most recent data, demands in 2050 would be in the range of 2.0 to 2.5 MGD.
3. In addition, an extrapolation of the trendline, which is shown in Figure 1-1 and enumerated in Table 1-2, produces the usage rate forecasts shown in Table 1-3.
Table 1-2 Purcellville Water Usage Trend
Year Usage Trend 1995 246,000 partial year record 1996 251,000 1997 276,000 1998 303,000 1999 344,000 partial year record
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Table 1-3 Purcellville Water Usage Forecast
Year Usage Forecast 2000 365,000 2005 500,000 2010 640,000 2020 920,000 2030 1,200,000 2040 1,480,000 2050 1,770,000
The average of these three analyses is 2.0 MGD in 2050. This matches exactly the previously published forecast data, which was based on the most detailed of the three analyses. Therefore, a demand of 2.0 MGD in 2050 will continue to be used.
Due to the decision by the Town of Round Hill not to participate in this study at this time, comprehensive figures for Western Loudoun County have not been reviewed. Therefore, Engineering Concepts, Inc. will proceed with analyses primarily focused on the demand of 2.0 MGD in 2050 for the Town of Purcellville and its Urban Growth Area.
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II. Neighboring System Review
1. Loudoun County Sanitation Authority
The 1996 Water and Wastewater Master Plan described a potential connection with the Loudoun County Sanitation Authority (LCSA) at a point on VA Route 621 about 3-1/2 miles south of the Leesburg town limits. The distance between this connection point and Purcellville is approximately 10.5 miles.
Water : LCSA currently does not have any water available. LCSA’s likely water sources would be their current sources, which are Fairfax City and Fairfax County, or the Potomac River.
Permitting : An implementation of this option would require LCSA to go through procedures similar to those required for other options in this document.
2. Hamilton
Water: The Town currently uses about 0.12 MGD and usage is expected to increase to approximately 0.32 MGD by about 2050. They operate 9 wells and are permitted to deliver 0.17 MGD.
3. Hillsboro
Water: Hillsboro has a system that can deliver 7,500 to 8,000 gallons per day to their 31 customers. Their source is a single 6.5 gallons per minute (gpm) well on Short Hill about 2,400 feet from Town.
Permitting: Other than potentially drilling a well to supplement their current source, they have no plans for expansion. They have not added a new customer since 1893. Yes that is eighteen ninety-three.
4. Leesburg
Water: Leesburg currently has sufficient water available for their own uses. They have more water available to them than they are currently using. However, many unused connections have been sold and if all of these were activated, the Town would possibly be at their limit. Their current capacity is 6.4 MGD about 70% of which comes from the Potomac River. Their current use is 5.0 MGD. They are in the process of upgrading their capacity to 10.0 MGD.
Permitting : There is currently an agreement between the Town and Loudoun County that Leesburg will not deliver water outside of their Town limits. This agreement currently limits Leesburg’s ability to participate in any regional system.
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5. Lovettsville
Water : Lovettsville currently has three wells and will soon drill a fourth. Total capacity is near to 0.1 MGD and they are using just about all of that.
6. Round Hill
Permitting: Not interested. No discussions.
This information is summarized in Table 2-1 below.
Table 2-1 Neighboring Water System Summary
Water Availability Permitting Difficulty Loudoun County Sanitation Authority No excess currently available. Would be similar to Purcellville’s process, but they would have the difficulties, not Purcellville. Hamilton Uses 0.12 MGD. Expected to increase to 0.32 MGD by 2050. Have 9 wells Hillsboro One 6.5 gpm well No plans for expansion. Leesburg Current use 5.0 MGD. Current capacity Town has agreed with Loudoun County to 6.4 MGD. Current capacity upgrade will not supply areas outside of Town limits. produce 10.0 MGD Lovettsville Total capacity near 0.1 MGD. They are using just about all of it. Round Hill No data Not interested in partnership at initial contact.
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III. Initial Source Review
Before initiating a detailed study, ECI reviewed a broad list of candidate water sources. The initial list included virtually all suggested options and was intended as a broad review of potential sources. This section of the report was submitted independently on August 5, 1999, and is reproduced here with minimal changes.
The list of candidate sources was reviewed by the Town’s Water Resources Committee, and provided a basis for the Committee to select three options for detailed study. The initial list included 12 options. These options were divided into the following categories for discussion purposes:
Ground Water & Springs 2 option categories Reservoirs 4 options River Connections Potomac 3 options Shenandoah 3 options
Three major issues were considered in the comparison of source candidates. These were the water quantity available, permitting and other administrative constraints, and cost. The permitting discussion refers to the relative degree of difficulty, frustration and time it might take to resolve all of the administrative, legal and other public requirements that must be completed before construction can begin.
Much of the information presented was taken from previous work done by others, and some is based on verbal communications. Cost estimates are conceptual in scope and are intended to facilitate a construction cost comparison between options. Cost estimates were not intended to be an engineer’s estimate. No cost estimates are included for additional finished water storage facilities nor are any costs included for upgrading the line from the existing treatment plant to Town.
Permitting difficulties are the most subjective of all the assessments presented herein. The evaluation presented was our best assessment of the comparative level of difficulty of permitting the options discussed. Even the simplest of the options could generate objections from individual citizens and interest groups.
A. Wells
Water: With an extensive groundwater search, up to 12 wells with a capacity of up to 60 gpm may be found. This would yield up to 1.0 MGD if they were run 24 hours per day. Based on health department requirements for 0.5 gallons per minute (gpm) of capacity for each equivalent residential connection (ERC), twelve 60 gpm wells would serve 1440 ERC’s. At a usage of 400 gallons per day (gpd) per ERC, this system would provide about 0.6 MGD. This results in well operations of about 13.3 hours per day.
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Multiple sites considered favorable for developing additional groundwater sources were selected. These generally lie in the areas northwest of Purcellville, along the South Fork of Catoctin Creek and near the water treatment plant and reservoir. There may also be some potential sites south of Town.
Permitting: The permitting process for groundwater options will be much simpler than any of the others cited in this report. The primary constraints will most likely be property ownership related.
Cost: A conceptual cost estimate for bringing approximately one half of the potential yield on line is approximately $1.2 million. The total investment to bring 0.6 to 1.0 MGD on line is conceptually $2.5 million. This is based on drilling approximately 26 exploratory wells, which could result in 12 final production wells as discussed above. The total estimate assumes approximately 5 treatment sites might be needed depending of course on the quality of the resulting water. The ‘initial’ costs for this option do not necessarily need to be taken in one large step. Search and development of wells could be implemented in much smaller steps than the other options described in this report.
1. New Springs
Water: A search for additional springs would involve field investigation. Search techniques would involve reviewing stream flows in branches with various sized drainage areas to detect watercourses that displayed disproportionately healthy flows. When these were identified, they would be traced to their sources. Physical travel with the guidance of USGS quad sheets should enable a rigorous search of the east slope of the Blue Ridge and possibly the east and west slope of Short Hill.
Permitting: The permitting effort would be comparable to that for wells, unless a source that justified an impoundment was located. Major regional issues should not be a problem. The focus will more likely be with the Health Department and individual property owners. Subsequent to the initial submittal of this section, the activities of the Loudoun County Groundwater Advisory Committee have indicated that there may be more regional interest in groundwater than was initially indicated.
B. Reservoirs
As a group, the various reservoir options have the most variety. Four different locations were reviewed. These are: the existing J. T. Hirst site, the Cooper Spring site, the Catoctin Creek site, and Sleeter Lake. The most significant differences between these options are the safe yield and the property ownership impacts. Safe yield is a function of the size of the natural drainage basin and the presence of significant springs in the watershed. To a lesser degree, another discriminator between these options is the physical location relative to the existing Purcellville facilities. This is a lesser concern for this category of sources because they are all located within a few miles of either the Town or the existing delivery system, and the differences among them will probably not
\Water Resource Study.doc 15 3/14/00 weigh heavily in a decision relative to these options. These variables are briefly summarized in the Table 3-1 below.
Table 3-1 Reservoir Summary Natural drainage Springs Property Ownership (sq. mi.) J. T. Hirst 1.3 Potts, Harris Town + 1 or 2 others Cooper Spring 1.3 Cooper Town + 1 or 2 others Catoctin Creek 17.5 Cooper, Unknown Copperhead, Potts, Harris Sleeter Lake 10.8 Several Oak Hill Properties
All of the reservoir options are located entirely within the Commonwealth of Virginia. This is a distinct advantage over the river options.
1. New Impoundment at Existing J. T. Hirst Reservoir Site
Water : According to the 1987 Reservoir Water Supply Study, water yields from the existing dam site could be increased by an additional 0.2 to 0.3 MGD depending on the final design.
Permitting : This option would involve a minimal number of property owners.
Cost: The initial cost estimate was based on an adjustment of the 1987 estimate. The adjustment yielded a cost of approximately $3 million in initial investment to construct the dam in today’s dollars. Subsequent investments could total an additional $1 million for transmission pipe upgrades.
2. Cooper Reservoir
The primary difference between the Cooper Spring and Hirst Reservoir sites are the optimization of the use of the site and yield to be obtained from these two sources.
Water : Anecdotal evidence indicates that there may be additional water available from this source. However, actual measurements obtained to date have not been able to verify this.
Permitting : This option would involve a minimal number of property owners.
Cost: The cost of providing an impoundment at this location would depend upon the quantity of water available. Costs for a comparable structure would be similar to those for a new impoundment at the J. T. Hirst Reservoir site.
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3. Catoctin Reservoir
Water : The 1996 Water and Wastewater Master Plan describes the safe yield of a proposed reservoir on the North Fork Catoctin Creek to be greater than 1.0 MGD.
Permitting : Permitting of this site will be much more controversial than the J.T. Hirst or Cooper Spring sites. The degree of difficulty relative to the potential river sources is difficult to quantify. Some aspects such as dealing with large-scale environmental issues might be reduced, but other issues on a more local scale will be greatly increased.
Cost: Cost estimates reviewed to date have been vague in origin and detail. However, total costs in the order of $14-16 million in current dollars have been discussed. If construction of some items were postponed to a later date, this project might be initiated for about $8 million.
4. Sleeter Lake
Water : Safe yield from Sleeter Lake has been variously reported between 1.0 and 2.2 MGD. There is no current consumptive use of water and none is currently planned. It is close to Purcellville, and relatively close to the existing treatment plant.
Permitting : Sleeter Lake is privately owned and is currently used for a minor amount of recreation and as an aesthetic amenity for the development around it. The owners are willing to discuss options. Permitting of this source might be simpler than any of the other surface water options. The Department of Health appears to have reduced its objections to this source, and according to the owners, orchard operations around the lake have ceased.
Cost: Conceptual cost estimates indicated that this option might be initiated for approximately $2 million. This would be to gain control of the source and connect it to the existing treatment and distribution system. To fully implement this option, an additional $6 million for treatment facilities should be anticipated.
C. River Connections
The river connections as a group have one very distinct advantage over the other categories, that being the quantity of water available. There is much more water available in the Shenandoah or Potomac Rivers than any of the other options.
The biggest disadvantage is that with this large quantity of water also comes a large interest from other parties. This will significantly impact the permitting process no matter which river option might be selected. While there is much water ‘available’ on a technical and hydrologic basis, making this water ‘available’ on a permit basis could be a challenge.
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Potomac River Connections
Three locations for a water line to the Potomac River are discussed: Lovettsville, Leesburg and US Route 340. As a group, connections to the Potomac River all face the hurdle of dealing with the State of Maryland. One consultant I conversed with, when asked, “Who in Maryland would we need to talk to regarding water withdrawal from the Potomac?”, jokingly replied, “The governor.” While said in jest, there is value in realizing that this is currently a controversial issue.
On a technical basis, there is sufficient flow in the Potomac at any of these locations to support a 2-MGD withdrawal for Purcellville. The differences between these options primarily revolve around the lengths of line to each location, the territory through which such a line would travel, and the resulting point of connection to the existing Purcellville system. Some of these differences are briefly summarized in the Table 3-2 below.
Table 3-2 Potomac River Connection Summary Pipeline Length Purcellville (miles) connection point Leesburg 10.4 Town Lovettsville 15.0 Treatment Plant Route 340 8.4 Cooper Spring
1. Potomac River at Leesburg
The potential Leesburg intake site is 10.4 miles from Town, near Whites Ferry. The potential pipeline route would go north of and west of Morven Park, and along Route 7 up to Route 287.
Water : As with all of the Potomac and Shenandoah River options, the physical availability of water is not a problem.
Permitting : Permitting of any of the Potomac River options will involve the State of Maryland, which could be a very significant issue.
Cost: The conceptual cost estimate for the Leesburg pipeline option includes a new water treatment plant as part of the initial investment. This is because the pipeline route enters Purcellville from the east while the existing treatment plant is northwest of Town. In all of the other pipeline options the treatment plant expense is postponed until the subsequent expenditures. Initial cost therefore could be in the order of $11 million dollars for the pipeline, and the pumping and treatment facilities. This would bring all water delivery facilities up to final configuration and no subsequent expenditures should be necessary.
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2. Potomac River at Lovettsville
A potential Lovettsville withdrawal site is 15.0 miles from the existing water treatment plant via Routes 287 and 9.
Water : As with all of the Potomac and Shenandoah River options, the physical availability of water is not a problem.
Permitting : Permitting of any of the Potomac River options will involve the State of Maryland, which could be a very significant issue.
Cost: Conceptual cost estimates for this option indicate that initial expenditures for a pipeline through Lovettsville could be in the order of $6 million for pipeline and pumping facilities. Subsequent expenditures could be on the order of an additional $6 million for treatment capacity expansion.
3. Potomac River at Route 340
A potential Potomac River withdrawal site is near US Route 340. It is 8.4 miles from Cooper Spring via Route 671.
Water : As with all of the Potomac and Shenandoah River options the physical availability of water is not a problem.
Permitting : Permitting of any of the Potomac River options will involve the State of Maryland, which could be a very significant issue.
Cost : Conceptual cost estimates for this option indicate that initial expenditures for a pipeline from the Potomac River near the US Route 340 crossing to the Cooper Spring area could be in the order of $4 million for pipeline and pumping facilities. Subsequent expenditures could be on the order of an additional $7 million for treatment and transmission upgrades.
Shenandoah River Connections
The potential Shenandoah River connections come with a different set of permitting issues. The advantage is that the State of Maryland will have a lesser interest; however, this may not necessarily be a zero interest as the Shenandoah is a tributary to the Potomac.
All three of these options could use the existing water treatment plant on Short Hill Road.
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Table 3-3 Shenandoah River Connection Summary Pipeline Length Purcellville Withdrawal State (miles) connection point Route 9 4.2 Cooper Spring WV Shannondale 3.6 J. T. Hirst WV Route 7 10.6 Treatment Plant VA
4. Shenandoah River at Route 9
This is the second shortest river pipeline option. It involves about one mile of pipeline within the State of West Virginia.
Water : As with all of the Potomac and Shenandoah River options the physical availability of water is not a problem.
Permitting : Use of a potential realignment of Route 9 within the State of West Virginia might simplify alignment issues, but the involvement of WVDOT with such an alignment would add another government agency to an already complicated process. An alignment along private property within West Virginia might be simpler. The inability to condemn property in West Virginia would require the cooperation of a willing seller of easements.
Cost: Conceptual cost estimates for this option indicate that initial expenditures for a pipeline from West Virginia via Route 9 to the Cooper Springs area could be in the order of $2.5 million for pipeline and pumping facilities. Subsequent expenditures could be on the order of an additional $7.5 million for treatment and transmission upgrades.
5. Shenandoah River at Shannondale
The most significant advantage of this option is that it is the shortest and most direct route between a Purcellville water facility and a major river.
Water : As with all of the Potomac and Shenandoah River options, the physical availability of water is not a problem.
Permitting : This is one of the two Shenandoah River options that involve an intake in the State of West Virginia. It also includes a crossing of the Appalachian National Scenic Trail (ANST). To cross the ANST, the National Park Service (NPS) has indicated that we will need to convince them that there is no other ‘practicable’ alternative for Purcellville to obtain water. Another disadvantage is Purcellville’s inability to condemn property in West Virginia.
To date, copies of the documents conveying rights in the ANST buffer strip from Purcellville to the NPS have not been obtained. There may be some loopholes in these
\Water Resource Study.doc 20 3/14/00 documents that retained sufficient rights for Purcellville, but this must be researched at the courthouse in Leesburg.
Cost: Conceptual cost estimates for this option indicate that initial expenditures for a pipeline from West Virginia to the Hirst Reservoir area could be in the order of $2.5 million for pipeline and pumping facilities. Subsequent expenditures could be on the order of an additional $7.5 million for treatment, storage and transmission upgrades.
6. Shenandoah River at Route 7
Water : As with all of the Potomac and Shenandoah River options the physical availability of water is not a problem.
Permitting : This is the only Shenandoah River option that remains within the Commonwealth of Virginia. This minimizes the complexities of the permitting process, but does not necessarily minimize objections or ensure success. The Chairman of the State Scenic River Advisory Board has expressed opposition to this option.
Cost: Conceptual cost estimates for this option indicate that initial expenditures for a pipeline from West Virginia via Route 7 to the existing water treatment plant area could be in the order of $5 million for pipeline and pumping facilities. Subsequent expenditures could be on the order of an additional $6 million for treatment, storage and transmission upgrades.
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Summary
Table 3-4 summarizes the significant decision parameters.
Table 3-4 Initial Source Decision Variables New Permitting Initial Ultimate Better Water Effort Conceptual Conceptual Options Availability Ranking Cost Cost (MGD) Estimate Estimate ($ million) ($ million) New Wells 0.6 1 1.2 2.5 * New Springs 2 Hirst Reservoir 0.4 4 3 4 Cooper Reservoir 0.1 4 3 4.5 Catoctin Reservoir 1.1 5 8 14 Sleeter Lake 1.6 3 2 8 * Pot.-Leesburg 4.0 8 11 11 Pot.-Lovettsville 4.0 8 6 12 Pot.-Route 340 4.0 8 4 11 * Shen.-Route 9 4.0 7 2.5 10 * Shen.-Shannondale 4.0 9 2.5 10 Shen.-Route 7 4.0 6 5 11 *
The subjective rankings of permitting difficulties were established with the following logic: a) New wells were assigned the simplest effort since primary efforts would be negotiations with individual property owners. b) New springs were assigned a slightly higher effort since larger sources might require some small impoundments. c) Sleeter Lake was ranked next most difficult due to negotiations with a potentially savvy owner and the interest of the Health Department in this source. d) Hirst and Cooper Reservoirs were assigned the next highest difficulty due to the need to construct a significant impondment. e) Catoctin Reservoir was next due to the much larger facility construction and the added interest that this would attract. f) All the remaining alternatives involve the rivers, which will attract regional or interstate attention. Simplest is the Shenandoah-Route 7 option that involves only the Commonwealth of Virginia. g) Next is Shenandoah-Route 9, which adds the State of West Virginia. h) Next are all the Potomac River options which involve the State of Maryland. i) Finally, the Shenandoah-Shannondale option includes both the State of West Virginia and a crossing of the Appalachian Trail outside of a road corridor.
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IV. Detailed Source Review
On August 31, 1999 the Town of Purcellville directed Engineering Concepts to investigate three options for water resources availability. These options were:
1) New wells.
2) The Mountain Resource, including: a) Cooper Reservoir b) New Springs c) Expanding Hirst Reservoir d) Piping water from Cooper Spring to Hirst Reservoir
3) Sleeter Lake
Subsequent direction from the Town indicated that efforts should concentrate on the groundwater (i.e. new wells) and Mountain Resource options. Investigations conducted relative to the Sleeter Lake option are included herein, but the level of detail is less than that presented for the other two options.
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V. Wells
A. Introduction
1. Study Objectives and Evaluation Criteria
The principal objective of the groundwater resource evaluation is to provide an assessment on the overall feasibility of developing groundwater sources to supplement the Town of Purcellville's existing public water supplies. The criteria for locating additional groundwater sources and assessing the long-term sustainable development potential of groundwater supplies relate to the yield of the geologic formations and structures underlying the study area and the long-term recharge potential of the aquifer. Other considerations on the feasibility of developing additional groundwater sources include site accessibility, well and treatment facility costs, water quality concerns, and the potential impacts of groundwater development on existing groundwater use and surface water resources.
2. Scope of Work Completed
The scope of work completed as part of the groundwater resource study included:
• Evaluation of the geology of the study area by reviewing published geologic maps and hydrogeologic reports prepared by the Virginia Division of Mineral Resources (VDMR) and the U.S. Geological Survey (USGS);
• Evaluation of data on existing private and public water supply wells supplied by Virginia Department of Health (VDH) and the Loudoun County Health Department (LCHD);
• Estimation of the groundwater recharge potential based on long-term climatological data, published stream baseflow calculations, soil maps, and surface water drainage maps;
• Identification of potential water quality concerns by reviewing available water quality analyses from area wells and identifying potential contamination sites through federal, state and county databases;
• Performing a bedrock lineament or fracture trace analysis of the study area by examining topographic maps, aerial photographs and digital elevation imagery; and
• Assessing the overall availability of groundwater resources within the study area and providing recommendations on areas considered favorable for further groundwater exploration and development.
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3. Location and Description of Study Area
The groundwater study area (Figure 5-1) incorporates the area within existing town limits, the designated Urban Growth Area (UGA) surrounding Purcellville, and a 3,000- foot wide strip along Purcellville's water line between Town and Purcellville's Hirst Reservoir located at the base of the Blue Ridge. The 1,220 acres of land owned by Purcellville that surrounds the reservoir and its three springs (Potts Spring, Harris Spring and Cooper Spring) are included in the groundwater study area. The entire study area encompasses approximately 10.7 square miles (6,840 acres). Analysis of geologic and hydrogeologic characteristics generally extended an additional one half mile beyond the study area boundary.
Western Loudoun County is situated in a broad valley bounded by the Catoctin Mountain to the east and the Blue Ridge to the west with Short Hill Mountain in the middle. The valley floor has low relief, characterized by gently rolling hills and eastward-draining stream valleys. Most of the study area lies within the South Fork of Catoctin Creek watershed. Portions of the study area near the reservoir drain to the North Fork of Catoctin Creek. Areas generally south of Route 7 drain to the North Fork of Goose Creek watershed. All of these drainage systems are part of the Potomac River Basin. Elevations range from over 1,600 feet on the Blue Ridge, over 1,000 feet atop of Short Hill Mountain, and approximately 500 feet along Catoctin Creek near Purcellville.
B. Groundwater Conditions
The feasibility of developing groundwater resources of suitable quantity and quality are controlled by numerous interrelated factors including: the water producing and water quality characteristics of the underlying bedrock formations; the presence of geologic structures favorable to groundwater development such as folds, faults or fracture zones; surface topography and drainage patterns; soil thickness and character; the configuration of the groundwater table; the amount of available groundwater recharge; and the effects of existing land usage on groundwater recharge and water quality. These factors are relevant to assessing the overall potential of developing suitable groundwater resources within the study area and are summarized in this section.
1. Geology
A detailed map of the geology found in the study area is presented as Plate 1. This map was obtained from Loudoun County's Office of Mapping and Geographic Information and is from the USGS Open File Report by Southworth and others (1999). The map was compiled from the 1:100,000-scale map of Loudoun County geology by Burton and others (1992) and several 1:24,000-scale geologic maps including the Purcellville quadrangle (Southworth, 1995) and the Round Hill quadrangle (McDowell
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Insert Figure 5-1
\Water Resource Study.doc 26 3/14/00 and Milton, 1992). A generalized geologic map and a geologic cross section of Loudoun County from Burton and others (1992) are provided as Figures 5-2 and 5-3. A detailed description geologic formation, structures and tectonic history found within the study area are provided in Appendix B.
Surficial Geology and Soils
Unconsolidated surficial deposits within the study area include alluvium, colluvium, soil and weathered bedrock. Alluvium is generally present along the flood plains of most streams and small tributaries in the area and includes a well to poorly stratified mixture of clay, silt, sand, gravel, and cobble. Coarse quartzite-boulder colluvium is present as a thin veneer on hillslopes and is concentrated in hillslope depressions and drainages by gravity or debris-flows and freeze-thaw processes. Springs are commonly found issuing from the toe of debris flows.
Most of the soil within the study area belongs to the Purcellville-Tankerville-Middleburg Soil Association (USDA, 1960). These soils form in upland low relief areas on crystalline metagranites and gneisses and primarily consist of well to moderately well drained, deep to very deep loamy soil. This deep weathering and formation of sandy permeable soils is due to the granitic parent material. Quartz and muscovite, both common minerals in granites, are highly resistant to chemical weathering and will remain in the saprolite or soil giving it its sandy texture. Other common minerals such as feldspar, biotite, and amphibole weather readily to form hydrated clays, which may be leached away to some extent, leaving void space in the saprolite residuum. This type of saprolite can be highly permeable and have the capability to provide excellent storage and groundwater recharge. Saprolite will generally be thickest in upland areas and is thin to absent on steeper slopes due to erosion.
2. Hydrogeologic Characteristics of Bedrock Formations and Structures
Because bedrock is generally impermeable, the capacity for bedrock to store and transmit groundwater is highly dependent on the density and interconnectivity of secondary void spaces or bedrock fractures present within the rock. Understanding the occurrence and hydrogeologic significance of bedrock fractures is an important aspect of groundwater exploration and development. The development of bedrock fractures is typically dependent on the lithology of the rock and the type of stresses exhibited on the rock mass to form the fracture permeability. Most of the rock types in the study area have the potential to develop fractures with good water-producing characteristics. In addition, the rock formations in the area have gone through multiple phases of compressional and extensional stresses likely resulting in the development of extensive fracturing.
Due to the massive, competent and coarse-grained nature of the granitic rocks, fractures within the Marshall Formation likely have the best water-bearing
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Insert Figure 5-2
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Insert Figure 5-3
\Water Resource Study.doc 29 3/14/00 characteristics. In these rocks, fractures will tend to be long and through going, will have greater aperture widths due to the rougher fracture surfaces, and will tend to remain open due to the general absence of clay-forming minerals. Additionally, near- horizontal sheet fractures attributed to removal of load stresses by erosion are often well developed in massive granitic rocks like the Marshall Formation. Sheet fractures and other less steeply dipping fractures such as cleavage and foliation partings tend to increase the hydraulic interconnections between bedrock fractures and may greatly improve the overall water-producing characteristics of these rocks. Also, shearing along the contacts between metagranite and metadiabase units often promotes fracture development. As such, these contacts often represent preferred groundwater pathways.
The overlying metasedimentary and volcanic rocks likely have highly variable fracture characteristics due to the diverse lithologies present. Fractures in the massive sandstone conglomerate and metabasalt units may have characteristics similar to the granitic rocks, open and through going. However, fracture densities may be low, potentially causing well yields to be highly variable in these rock types. The finer- grained sequence of the upper Swift Run and lower Catoctin Formations should have more favorable water-producing characteristics. The compositional and textural differences between the lithologies may promote fracture development and the presence of carbonate interbeds may form highly transmissive, solution-enhanced groundwater pathways.
The Short Hill fault may have a significant positive influence on the water-producing characteristics of the bedrock aquifer due to the potential increase in fractures along this surface, the compositional and textural differences of the rock on either side of the fault, and the regional extent of the fault. The Short Hill fault may act as a permeable zone, collecting and draining groundwater from other interconnected fractures systems found along its length.
3. Bedrock Lineament Analysis
Lineament or “fracture trace” analysis is a technique developed in the 1960s and 1970s for mapping potential bedrock fracture systems using remote sensing techniques (Lattman and Parizek, 1964; Siddiqui and Parizek, 1971). When used in conjunction with detailed stratigraphic and structural geologic mapping, this technique can assist in locating high-yielding wells. Fracture zones are loosely defined as steeply dipping zones of increased fracture density. This increase in fracture density may relate to stratigraphic parameters, such as bed spacing and lithology; or to brittle deformation fabrics such as joints, faults or folds. Because fracture zones are typically less resistant to weathering they are often expressed as natural topographic lineations or as tonal lineations due to soil moisture or vegetation alignments. These linear features can be identified by analyzing topographic maps, aerial photographs, and radar or satellite imagery. Identification of lineaments on several different types or scales of imagery, and correlation of lineament orientations with regional geologic trends or outcrop-scale bedrock fracture systems help to provide confirmation that mapped lineaments represent manifestations of underlying bedrock fracture zones.
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For this study, lineaments were identified by examining USGS topographic maps, 1:24,000-scale black and white, 1:40,000-scale color infrared aerial photographs, and processed digital elevation model (DEM) data. Photograph sets had stereo coverage of the study area allowing three-dimensional viewing. Shaded relief maps of large areas were constructed from the DEM data, allowing long (>1 mile) lineaments to be easily identified.
DEM Lineament Analysis
Figure 5-4 illustrates a color shaded relief map of the six 7.5-minute (1:24000 scale) quadrangles covering much of western Loudoun County. The area shown in this figure encompasses a 346.5 square mile area and includes portions of the Triassic Basin and the Blue Ridge anticlinorium between Catoctin Mountain and the Blue Ridge. Sixty-eight DEM lineaments were identified. Rose diagrams of DEM lineament orientations reveal four prominent DEM lineament sets: two orthogonal (perpindicular) sets oriented approximately East-West (88 o, read as east of north) and North-South (178 o), a northwest-trending (126 o) set that cuts across regional geologic strike, and a subordinate set of strike-parallel lineaments (20 o). The average length of the mapped DEM lineaments is 3 miles, the minimum and maximum lengths are approximately 1 mile and 7.5 miles, respectively. The longest DEM lineaments trend northwest and west approximately perpendicular to the strike of the bedrock.
The north and east-trending lineaments occur mostly in the northern portion of the area shown on Figure 5-4 and coincide with a slight northward bend in Catoctin Mountain and the Blue Ridge. This lineament orientation also correlates well with Mesozoic tectonic fabrics (Jurassic diabase dikes and transverse faults). Northwest-trending lineaments coincide with compressional tectonic fabrics, which are commonly transverse to regional geologic strike. Northeast-trending lineaments are parallel to geologic strike and may relate to stratigraphic influences such as differiental weathering of late proterozoic dikes, or to structural fabrics related thrust faults, shear zones, cleavage, normal faults or along axial plane surfaces of regional folds. The excellent correlation between DEM lineament orientations and known geologic structures and processes strongly suggests that most of the lineaments are related to underlying fractures zones, facilitating the use of lineaments as a groundwater exploration tool.
Photolineament Analysis
Photolineaments were identified by examining USGS topographic maps, 1:24,000-scale black and white aerial photographs and 1:40,000-scale color infrared aerial photographs. Approximately 200 photolineaments were identified within the study area (Figure 5-5). Generally, photolineaments coincide with stream valleys, topographic
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Insert Figure 5-4
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Insert Figure 5-5
\Water Resource Study.doc 33 3/14/00 depressions and soil/vegetative tonal anomalies occurring across farmland and pastures. Identified photolineaments range between 550 and 8,000 feet in length and average approximately 2,200 feet. Rose diagrams provided in Figure 5-5 indicate that the dominant photolineament orientations in decreasing frequency are 118 o, 12 o, 102 o, 154 o, 34 o and 66 o. Many of these photolineament directions can be correlated with regional tectonic fabrics indicating that most are manifestations of actual subsurface fracture zones.
4. Occurrence and Movement of Groundwater
The occurrence and movement of groundwater in the subsurface can be related to the hydrologic cycle. The hydrologic cycle refers to the constant movement of water in the atmosphere and on and beneath the surface of the earth. It begins with precipitation, the source of all surface water and groundwater supplies. As precipitation occurs, most of the water returns to the atmosphere by evaporation and transpiration from plants. The remainder flows overland to nearby streams or infiltrates past the root zone and eventually reaches the zone of saturation (water-table aquifer). From here, groundwater will move along specific flow paths governed by horizontal and vertical hydraulic gradients towards areas of discharge at springs, streams, lakes and wetlands.
Groundwater flow systems are often categorized as local, subregional and regional based on the length of the flow path between recharge and discharge areas (Toth, 1963; Freeze and Cherry, 1979). Local flow systems are characterized by short flow paths within shallow aquifers with small-scale topography usually controlling the location of recharge and discharge areas. Within the study area, a local flow system likely exists within the saturated portions of the unconsolidated surficial material and the upper weathered zone of the bedrock. Here, groundwater flow can be inferred from surface topography and generally flows in directions similar to surface water runoff, that is from topographically high areas to low areas. Recharge occurs directly by infiltration of precipitation and downward leakage from streambeds and on-site waste disposal systems. Discharge occurs as seepage or springflow into small streams and lakes or by direct evapotranspiration in wetland or shallow water table areas. The major streams in the area such as Catoctin Creek and Goose Creek likely act as discharge boundaries to the local flow systems.
Subregional and regional flow systems are characterized by generally long flow paths within deeper aquifers and are controlled by large-scale topographic features like the Blue Ridge, and Short Hill Mountain and the larger streams to the east. These flow systems comprise the groundwater flow regime within the fractured bedrock aquifer below the local flow system. Groundwater flow within the bedrock is controlled by the frequency and orientation of bedrock fractures, but for the most part will move from surface water and interbasin divides (recharge areas) towards areas of discharge along the major streams and rivers. Within the deeper portions of the bedrock, groundwater flow is predominantly towards the east and northeast towards the South Fork of the Catoctin Creek and southeast towards the North Fork of Goose Creek (Benegar, 1996). Groundwater flow paths within the deeper regional flow systems likely originate as recharge along the major groundwater divides such as Short Hill Mountain and the Blue
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Ridge and presumably flow under smaller basins defined by local flow systems. It should be noted that groundwater flow systems are generally hydraulically interconnected with each other with low hydraulic conductivity material (e.g. clay saprolite, unfractured bedrock) only locally confining or isolating groundwater flow.
5. Groundwater Recharge Estimates
Groundwater recharge from infiltration of precipitation can vary significantly in a region or in a single watershed. Recharge rates have been shown to vary from 0 to 25 inches per year within a single watershed in relatively similar Piedmont settings (NJGS, personal communication). Most of the variability in recharge can be attributed to factors related to climate, surficial geology and land use and cover. Some of the more important causes of variation in groundwater recharge rates are the permeability and thickness of surficial soils, topography, the type vegetative cover, local climate (precipitation, evaporation), and amount of impermeable surface coverage. Impervious area is generally associated with urban development and includes streets, roofs, driveways and parking lots. Nearly all precipitation that falls on these areas either runs off or evaporates directly. The runoff may be routed to retention basins where some portion may become groundwater recharge. Within the study area, recharge rates are expected to be highest in areas covered by forest, cultivated fields or pastures and residential developments on greater than 3-acre lots; and in areas underlain by thick sandy soils developed on granitic bedrock. Groundwater recharge rates are expected to be lowest in more densely developed areas of downtown Purcellville and in areas underlain by hydric soils, wetlands, surface water bodies or areas that have shallow or exposed bedrock outcrops.
The amount of water that recharges a groundwater system is generally estimated by calculating a water budget for an area or by examining stream flow hydrographs and separating out the contribution from groundwater discharge called baseflow. Using a water budget approach, the net recharge to the groundwater system is calculated using the following equation: groundwater recharge = precipitation - surface water runoff - evapotranspiration
Using the water budget approach, studies from Piedmont regions in Maryland and Virginia indicate that about 70 percent of the total precipitation is lost to evapotranspiration, 7 percent is lost as surface water runoff and the remaining 23 percent recharges the groundwater system (from Richardson, 1982). Estimates of effective groundwater recharge in these studies range from 8.5 to 11.3 inches per year. Assuming that the average annual precipitation in the study area is 42.15 inches per year (based on 30-year normal precipitation at NOAA's gaging station in Lincoln, Virginia), the average recharge rate would be about 9.7 inches per year for the Purcellville area.
Stream baseflow measurements are a more direct way of assessing groundwater recharge in an area. Under this method, it is assumed that the mean baseflow in a stream is equal to groundwater recharge. This method produces average recharge
\Water Resource Study.doc 35 3/14/00 rates within an entire watershed reflecting variations in climate, geology, topography and existing land use and land cover conditions during the period of streamflow gaging. Based on continuous-record streamflow gaging stations on Catoctin Creek at Talyorstown from 1973 to 1984 and on Goose Creek at Middleburg from 1967 to 1984, the groundwater recharge rates were estimated at 9.18 and 10.72 inches per year, respectively (Nelms and others, 1995). Weighted for watershed drainage area, this represents an average groundwater recharge rate of 10.1 inches per year over the 212 square mile area encompassing these two watersheds.
Based on these studies, 10 inches of precipitation is considered a reasonable groundwater recharge rate on an average annual basis. This is equivalent to a recharge rate of 475,000 gallons per day per square mile. The groundwater study area, which encompasses approximately 10.7 square miles, therefore receives approximately 5.1 million gallons per day of groundwater recharge on an average annual basis. The estimated amount of groundwater recharge inclusive of the South Fork Catoctin Creek watershed area upstream of Purcellville is roughly double this amount or approximately 10 MGD.
6. Inventory and Description of Area Wells
An inventory of drinking water wells has been compiled from the EPA Storet database and the Loudoun County Health Department. The Loudoun County Health Department (LCHD) maintains a very large database of wells drilled within the County. The well database obtained from LCHD has records on over 22,000 wells. However, the database has not been updated since 1995 and well yield information is available on only about 4,900 wells. Statistical analysis of this data indicates that wells within Loudoun County yield from less than 1 gpm to 1,000 gpm and that the average well yield is 19 gpm. Approximately 10 percent of the wells have a yield of 50 gpm or more, and 2 percent of the wells have a yield of 100 gpm or more. Well depths generally range between 200 and 500 feet with an average depth of approximately 325 feet. Overburden thickness is highly variable with a median value of 26 feet.
The location and yields of water supply wells within the study area are shown on Plate 1 and a thematic map of well yields is shown on Figure 5-6. There are numerous wells in the area that have reported well yields in excess of 100 gpm including wells at the following subdivisions: Brown's Farm (150 gpm), Jonella Farm (100+ gpm), Round Hill Sleeter Lake property (217 gpm) and the Marsh Property (250 gpm). At the Sleeter Lake property, eighteen test wells were drilled with blowtest yields ranging from 3 gpm to 217 gpm with six wells that yielded more than 100 gpm. The combined long-term yield of the eleven wells that were converted to production wells at the property was 977 gpm for an average well yield of 88 gpm. At the Marsh Property, twenty-one test wells were drilled with yields ranging from 2 gpm to 250 gpm. These wells are located within the designated study area.
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Insert Figure 5-6
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7. Groundwater Quality Issues
Potential sources of contamination may include point sources such as underground fuel storage tanks, landfills, wastewater treatment facilities, and industrial sites; and non- point sources such as residential septic systems, roadway and parking lot runoff, and agricultural fertilizers and pesticides. A comprehensive search of the EPA Envirofacts database and the VDEQ Leaking Petroleum Storage Tank (LPST) database were completed to identify potential sources of groundwater contamination within the study area.
The EPA database includes an inventory of EPA regulated facilities that store, generate or treat hazardous waste in compliance with state and federal regulations. A total of 19 facilities were listed on the EPA database with a Purcellville address (Table 5-1). Most facilities have either a VPDES discharge permit, a RCRA general facility permit as small quantity hazardous waste generator or have general air emissions permits. One of the facilities, Creative Urethanes Inc., has a reported toxic release. Three of the permitted facilities are laundry or dry cleaning facilities. Several others are vehicle repair facilities.
A review of the VDEQ LPST database indicates that there are 27 facilities or residences with reported leaking tanks. It is not known which of these have reported actual groundwater contamination. Most of the facilities are located along or near Main Street in Purcellville.
Overall, the potential for contamination is considered to be moderately to very high in the Purcellville area due to the presence of numerous known pollution sites and the presence of contamination in the Town of Purcellville's wells. Methyl tertiary butyl ether (MTBE), a gasoline additive, is one of the main contaminants found in area drinking water supplies including Purcellville, Round Hill and Hamilton. Due to the high number of potential sources (leaking petroleum storage tanks) and its high solubility, this compound may be relatively pervasive throughout each of these town centers. Water samples from any exploratory testing program should be analyzed for MTBE.
Other possible water quality concerns in this area include bacteria and nitrate, which may occur locally in areas of high septic field concentration and thin soil cover. Naturally occurring iron and manganese are also found in many of the area wells. Of the 63 wells that had reported iron concentrations in the EPA Storet database, 70 percent meet the secondary drinking water quality standard of 0.3 mg/l. The median iron concentration was 0.16 mg/l.
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TABLE 5-1 LIST OF EPA REGULATED FACILITES AND STATE LPST SITES
EPA REGULATED FACILITIES VA0001531854 Aire-Flo Inc. Rt. 3 Box 65d Purcellville VAD988192555 7-Eleven #24539 Rte 7 & Lincoln Way Purcellville VAD064870892 AT & T Long Lines Equipment Eng Route 1 Box 141 Purcellville VAD980720163 Chesapeake & Potomac Telephone Co Main St Purcellville VAD049928823 Creative Urethanes Inc 310 N 21st St Purcellville VA0000829671 Emerick Elementary School Route 1604 Purcellville VA0002344075 Festival Cleaners 748 East Main St Purcellville VA0000829648 Lincoln Elementary School Route 722 Purcellville VA0000707521 Loudoun Laundry Inc. 140 South 120th St. Purcellville VAD023885528 Loudoun Truck Center 631 W Main St Purcellville VA0000829689 Loudoun Valley High School Route 287 Purcellville VA0000835082 Loudoun Veterinary Service Inc 1043 East Main Street Purcellville VA0000203000 Purcellville Cleaners 609 C E Main St Purcellville 7299294 Purcellville Sewage Treatment Plant Purcellville VA0000830919 Real Tool Inc. 210 21st Street Purcellville VAD981106206 Terrys Body Shop 750 E Main St Purcellville VA0001131549 TLC Collision Inc 801-A 9th St N Purcellville VAD981935810 Towe & Johnson MD Medical Office 441 East Main St Purcellville 7607360 Town Of Purcellville Water Treatment Plant Purcellville VADEQ LPST DATABASE 81-0089 Whitmore & Arnold Fertilizer Purcellville 87-0094 Whitt Lowe (SLS) Lowe residence Purcellville 87-0801 Lynn Adams Seed Co. Purcellville 87-9991 Purcelleville Safeway Purcellville 89-1095 Marsh Residence Route 690 & 711 Purcellville 90-0348 Schonder Property 701 West Main Street Purcellville 90-0981 Union '76 731 Main Street Purcellville 90-1302 Purcellville Motors (XREF 90-1505 Route 7 Purcellville 90-1505 Schonder Property 701 West Main Street Purcellville 91-0760 WACO Oil (SLS) 860 East Main Street Purcellville 91-0982 Loudoun Laundry P. 0. Box 216 Purcellville 91-1186 Hessick, Inc. (SLS) East “O” Street Purcellville 91-1431 Texaco (SLS) 760 East Main Street Purcellville 91-1956 Whetsell Property 901 West Main Street Purcellville 92-0016 Browning Equipment, Inc. 800 East Main Street Purcellville 92-0111 MotorCar Garage East Main Street Purcellville 92-1721 Unocal 76 731 Main Street Purcellville 93-1144 Merit Construction 210-B North 21st Street Purcellville 93-2161 Werner Residence Route 1, Box 650 Purcellville 94-0299 Town of Purcellville 141 East Main Street Purcellville 94-1443 Old National Building & Supplies 630 West Main Street Purcellville 95-4261 Resnick Residence 341 Nursery Avenue Purcellville 96-3013 First Virginia Bank 115 East Main Street Purcellville 96-3173 Hessick Oil Facility (abandoned) “O" Street Purcellville 97-3062 Nichols Residence 311 Nursery Avenue Purcellville 98-3567 Shell Station (former) 118 Main Street Purcellville 98-3581 Loudoun Milk Transportation 631 West Main Street Purcellville
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C. Groundwater Availability
Based upon the findings of our preliminary groundwater investigation, HSI GeoTrans believes that the overall potential of developing additional groundwater resources for the Town of Purcellville to be excellent. The following key aspects of the groundwater conditions within the study area support this assessment:
1. Gentle topography and the presence of deep permeable soils promote the infiltration, storage and recharge of groundwater.
2. The bedrock has water-producing characteristics favorable for the development of transmissive, open and through-going fractures.
3. There is a high density of lineaments whose orientations correlate with tectonic fabrics. This indicates a well-developed, highly interconnected bedrock fracture network.
4. The Short Hill fault provides a potentially favorable target for developing high- yielding wells.
5. An inventory of water supply wells within the study area indicates that relatively large well yields commonly in excess of 100 gpm and up to 250 gpm are possible. Some of the highest reported yielding wells occur along the Short Hill fault near Purcellville's water transmission line.
6. The estimated groundwater recharge within the limits of the study area is approximately 5 MGD. The estimated annual recharge from precipitation inclusive of the entire watershed upstream of Purcellville is approximately 10 MGD.
7. The water transmission line between Purcellville and the Hirst Reservoir provides an excellent opportunity to develop groundwater resources over a large geographic area with minimal additional infrastructure.
Sustainable Yield Estimates The sustainable or safe yield of a single well or an entire groundwater basin is generally defined as the amount of water that can be withdrawn without producing an undesirable result such as impact to existing groundwater users, reduced streamflow, degradation of water quality, or land subsidence (Freeze and Cherry, 1979; Alley and others, 1999). Withdrawing groundwater from an aquifer is balanced by: 1) a lowering of hydraulic head locally in the aquifer (drawdown cone) as water is removed from storage, 2) an increase in recharge to the aquifer, 3) a decrease in the rate of discharge from the aquifer to streams and wetlands, or some combination of these three. Determining the sustainable yield generally requires withdrawing groundwater and monitoring the effects of this pumping on the groundwater flow regime or constructing a groundwater flow model to predict the potential consequences of various pumping scenarios. Generalized estimates of sustainable yield can also be made using a simplified water budget analysis.
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Because of the favorable groundwater conditions and the ability to develop new groundwater supplies over a large mostly rural area with relatively low infrastructure costs, we believe that it will be feasible to develop 15% to 25% of the total estimated groundwater recharge (5 mgd). This would be equivalent to a total sustainable yield of approximately 750,000 to 1,250,000 gallons per day from groundwater sources. This estimate may be low in that it does not account for increased aquifer recharge due to downward leakage from the water table aquifer and inflow from areas outside of the study area. However, this estimate does not account for periods of long term drought such as was experienced in Loudoun County last summer. Using a drought recharge rate of 6 inches per year (Memorandum from Jeffrey Widmeyer, LCDH dated February 27, 1989), under worst-case conditions the estimated sustainable yield would be 450,000 to 750,000 gallons per day. This range probably should be used for planning purposes by the town in determining the overall feasibility of developing groundwater as a public water supply.
Ultimately, the amount of groundwater available for development will depend on successfully locating high yielding wells and demonstrating sustainable capacity through long term aquifer testing and monitoring of potential impacts to existing groundwater users and surface water bodies. Potential drawdown impacts to existing groundwater users should be minimal due to the relatively rural setting found throughout much of the study area. Potential impact to stream baseflow should not be significant during non- drought conditions since the potential reductions in baseflow from groundwater pumping would likely represent less than 7 percent of the total mean flow in Catoctin Creek (based on capturing 15% of available groundwater recharge and that groundwater represents an estimated 43% of total stream flow). During severe drought conditions groundwater pumpage could have a more significant impact on stream baseflow.
D. Recommended Groundwater Exploration Areas
The study area was subdivided into six general areas where additional groundwater exploration and possible resource development were considered. The first four potential groundwater exploration areas are considered favorable to groundwater development due to the presence of several of the hydrogeologic factors that are known to have a positive influence on the development of high-yielding water supply wells, while minimizing potential water quality concerns relevant to the study area and potential impacts to existing groundwater users. The last two are not considered feasible for the development of groundwater resources due to water quality concerns and the greater likelihood of impacting existing wells. They are reviewed in this discussion as a matter of completeness and for comparison with recommended areas.
The groundwater exploration areas were ranked on the basis of hydrogeologic factors, potential water quality concerns (sources of pollution), potential impact to nearby wells, proximity to infrastructure and property access/ownership considerations. A matrix summary of these considerations and a ranking of recommended groundwater explorations areas from most favorable to least favorable is presented in Table 5-2. Figure 5-7 illustrates the general location of the recommended groundwater exploration
\Water Resource Study.doc 41 3/14/00 areas (GWEA’s 1 through 4). The advantages and disadvantages of each groundwater exploration area are discussed below.
1. Short Hill Mountain - This area is located at the base of Short Hill Mountain near the water treatment plant and along portions of the finished water line. This area is considered the most favorable hydrogeologically with wells targeting fracture- correlated lineaments and the Short Hill fault at depth. Well yields in the range of 100 to 250 gpm can be expected based on existing well records. The area is rural with few groundwater users and no known contamination sources. Water supplies could be pumped directly into the raw or finished water lines depending on treatment requirements. Property access is considered moderate due to potential future growth pressures.
2. Purcellville's Hirst Reservoir - This area is not as favorable hydrogeologically but the land is owned by the town, undeveloped and well protected. Operational considerations are considered poor to moderate due to the distance from the treatment plant. Property access is considered good due to ownership. Groundwater recharge estimates for this watershed are nearly 1 mgd, a portion of which discharges to the springs and the reservoir. Wells would serve to capture and develop a larger proportion of the available groundwater resources in this area. The wells would provide a raw water source to the treatment plant. Separate treatment facilities would not be required at the well site(s).
3. Reservoir to WTP - The area along the raw water line between the Hirst Reservoir and Purcellville's water treatment plant should also be considered. The potential well yield in this area is considered to be moderately good. The area is rural with few groundwater users, no known contamination sources and relatively slow growth pressure. Wells would be developed near the existing water line providing additional raw water supplies to the treatment plant. Expansion of the treatment plant may be required to accommodate the additional raw water sources or a finished water line may be needed to convey treated groundwater sources to the finished water transmission line.
4. Northern UGA - The northern section of the Urban Growth Area is considered favorable hydrogeologically and is believed to be a sufficient distance from known contamination sources located within the town limits. Additional protection may be provided by Catoctin Creek, which may act as a groundwater divide, potentially preventing groundwater movement to the north. Operationally, the wells would be relatively close to town and would be connected directly into finished water lines. However, at this point there is no infrastructure in this area. Property access is considered poor due to the relatively strong future growth potential in this area which may make it difficult to obtain access agreements.
5. Southern UGA - The southern section of the Urban Growth Area is considered favorable hydrogeologically but poor to moderate due to potential water quality concerns and potential impact to existing groundwater users
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6. Town Limits - Groundwater development within the town limits is considered to be poor due to groundwater contamination, potential interference with existing town wells and access to available property. Operationally, this area is considered to be good due to proximity to existing infrastructure. However, overall, additional groundwater development within the town limits or in the southern UGA should not be considered due to water quality concerns and potential drawdown impacts to residential developments south and southwest of the town limits and with the Town of Hamilton's wells to the east. Furthermore, Purcellville's undeveloped Main Street well should not be developed and the town's existing Main Street wells should be phased out of operation over time as new water sources are developed due to high operational costs of these wells.
Each recommended area has multiple potential test well sites. The location and ranking of specific test well sites should be based on further site-specific investigations including detailed geologic mapping and surface geophysical investigation to confirm and more accurately locate water-bearing fracture zones in the subsurface, delineation of wellhead protection areas and identification of potential contaminant sources, and an exploratory test well drilling program.
E. Costs of Development
A budgetary cost estimate has been prepared for the siting and installation of groundwater supply wells and designing and building the associated infrastructure to treat and/or interconnect the water sources. In general, costs include initial field investigations and exploratory test well drilling; construction, testing and permitting of production wells; design and construction of pump house with chlorination and iron/manganese treatment facilities; and design and construction of water supply lines and interconnections. Property acquisition costs are also included, although these cost are highly uncertain.
Cost estimates are based upon an assumption of drilling of a total of 15 test wells; 8 production wells with a capacity of 65 gpm each, at an average distance of 400 feet from exiting lines; and 4 well houses. Based on these assumptions a development cost of approximately one million dollars can be anticipated. Such development would be anticipated to produce approximately 0.42 MGD based on 13.3 hours per day of operation. Note, as cost figures are reviewed, some refinement of cost estimates may occur before final report is submitted.
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Insert Figure 5-7
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Table 5-2 Groundwater Exploration Area Evaluation Matrix Town of Purcellville Water Resources Study
Potential Potential Impact to Area Potential Potential Water Quality Groundwater Operational Property Groundwater Well Yield Concerns Users Considerations Access Exploration Area (Weight 4) (Weight 4) (Weight 3) (Weight 2) (Weight 2) Total Ranking Short Hill Mountain 3 3 2 3 2 40 1 Reservoir 1 3 3 2 3 35 3 Reservoir to WTP 2 3 3 2 2 37 2 Northern UGA 2 2 2 2 1 28 4 Southern UGA 2 2 1 2 1 25 5 In Town 2 1 1 3 1 23 6
Rating Weighting Factors 3 - Good 4 - most important component 2 - Intermediate 3 - 1 - Poor 2 - 1 - least important component
Potential Well Yield - Poor (<50 gpm), Intermediate (50-100 gpm), Good (100-250 gpm) Potential Water Quality Concerns - Poor (nearby contaminant sources), Intermediate (sources far away), Good (no known sources) Potential Impact to Groundwater Users - Poor (nearby wells or high density), Intermediate (wells farther away or low density), Good (no wells or very low density) Operational Considerations - includes distance to wells, proximity to existing infrastructure, source used as finished or raw water, safety Property Access - Poor (area developed or has potential of being developed), Intermediate (area not developed), Good (existing property)
F. Conclusions
HSI GeoTrans has completed a preliminary evaluation on the feasibility of developing groundwater resources to meet or supplement future water supply needs for the Town of Purcellville in western Loudoun County, Virginia. The primary objective of this portion of the study was to assess the overall availability of groundwater resources within the study area. A secondary objective was to identify favorable areas where additional groundwater exploration should occur. The conclusions reached from this investigation are as follows:
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1. The overall potential of developing additional groundwater resources for the Town of Purcellville is believed to be excellent due to favorable hydrogeologic conditions and the large mostly rural area where groundwater resources may be developed with minimal additional infrastructure.
2. A total of approximately 200 photolineaments and 68 DEM lineaments were identified within the study area through remote sensing analysis. The dominant lineament trends correlate well with tectonic bedrock fabric orientations strongly suggesting that most mapped lineaments represent true manifestations of underlying bedrock fracture zones.
3. An inventory of water supply wells indicates that relatively large well yields in excess of 100 gpm are possible within the study area. The reported highest yielding well in the area is rated at 250 gpm and is located within the study area near Purcellville's finished water transmission line.
4. The estimated groundwater recharge potential for the study area is approximately 5 million gallons per day assuming an average recharge rate of 10 in/yr and 3 million gallons per day assuming a drought recharge rate of 6 in/yr. Based on worst-case drought recharge estimates, it is believed that 450,000 to 750,000 gallons per day could be developed by the Town of Purcellville without significant impact to area groundwater users or surface water flow.
5. Four potential groundwater exploration areas have been identified. These areas have been selected due to the presence of favorable conditions for developing high- yield water supply wells, low contaminant threat and low potential impact on existing groundwater users. Each recommended area has multiple potential test well sites. The location and ranking of specific test well sites should be based on further site- specific investigations including a surface geophysical investigation and site accessibility.
G. Recommendations
The overall potential of developing groundwater supplies to meet or to supplement Purcellville's future water supply needs is excellent. It is estimated that up to eight production well facilities will be needed to provide up to 0.42 MGD of system capacity. This is based on the assumption that each production well facility would have an average yield of approximately 65 gpm. This is considered reasonable for the study area. It is assumed that each well facility would be pumped up to 13 hours per day based on current VDH regulations. To help insure the successful development of groundwater resources it will be necessary to conduct more detailed hydrogeologic studies within each of the recommended groundwater exploration areas. These studies should include the following recommended phases:
1. Conduct detailed geologic mapping to refine and/or verify geologic contacts, analyze geologic structures such as folds, faults and bedrock fracture sets, and to confirm
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mapped lineament locations and origin. Geologic cross sections will be constructed to show the orientation and depth of favorable hydrogeologic features such as faults or formation contacts.
2. Identify specific properties that are favorable for developing groundwater resources. This will be accomplished by generating electronic overlay maps of bedrock geology, mapped lineaments, surface water features, topography, soils, well yield, water quality and potential contaminant sources (depending on data availability) onto digital tax maps. This will provide a graphical means to evaluate the data and help to discern specific trends in well yields and water quality. Specific groundwater exploration areas will be selected and the owners of the properties will be identified.
3. Conduct surface geophysical surveys to confirm the presence or absence of a water-filled fracture zones and to precisely locate the orientation and dip of these fracture zones at depth. Based on the results of the geophysical studies, specific exploratory drilling sites will be identified and ranked.
4. Prepare specifications and solicit bids from qualified well drillers. The selected driller will then drill exploratory test wells and the yield and water quality will be assessed at each site. Selected test wells will be developed into public community water supply production wells.
5. Develop a source water protection plan for each new groundwater source.
6. Develop a groundwater level and streamflow monitoring program to monitor the long-term affects of the groundwater withdrawal.
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VI. Mountain Resource
The majority of the effort for this study has been devoted to the potential development of the Mountain Resource. This option, as presented to ECI by the Town, included four different sub-options. These were:
1) Cooper Reservoir 2) New Springs 3) Expanding Hirst reservoir 4) Piping water from Cooper Spring to Hirst Reservoir
These four sub-options can be discussed in terms of two variables: 1) the water source and 2) how to capture that source. Both of these components significantly affect the quantity of water available to the Town.
A. Sources
The four sub-options involve three water sources. These are:
1) Harris and Potts Springs and the watershed of Hirst Reservoir 2) Cooper Spring and its watershed 3) Other springs and their watersheds
Harris and Potts Springs
All flows from Harris and Potts Springs currently go through the Hirst Reservoir. Additional development of this source could be accomplished primarily through the expansion of storage capacity at or near the Hirst facility. Based on the minimal flow information available, the previously published safe yield of 0.25 MGD appears to be reasonable.
Cooper Spring
The minimal flow information available at this spring indicates that the average flow may be on the order of 0.12 MGD. Records from the existing pipeline from this spring indicate that average usage over the past four years has been 0.07 MGD, but this has risen to 0.09 MGD in 1999. Hydraulic calculations indicate that the existing pipeline, if it is in reasonably acceptable condition, can convey a major proportion of the treatment plant capacity from Cooper Spring when that much water is available.
Copperhead Spring
There is absolutely no flow information available for Copperhead Spring. Observations conducted on October 5, 1999 indicated a very minimal discharge from this spring. This discharge may of course vary with time and groundwater conditions.
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Cooper 2 Spring
This is a previously undocumented spring. Its source has not been located, however field observations conducted on October 5, and November 12, 1999 indicated that flows from this spring and other seeps downstream of Cooper Spring might combine to be on the order of 0.2 MGD. Once again, reliance on this minimal data is not advisable.
B. Water Quantity
The quantity of water available from a capture point has two different origins. First, and most significant, is the flow from the springs. Second is runoff from rainfall in the watershed upstream of the capture location. The runoff from rainfall is relatively easily quantified because rainfall is generally consistent within a similar geographic area and related streamflow records are readily available. This enables runoff flows to be statistically estimated.
The amount of water available from the springs is much more difficult, or perhaps, impossible to estimate. This is because each spring is individually unique and data from one cannot be extrapolated to another. To forecast the availability of water from any specific spring, a significant amount of data must be available for that particular spring. Unfortunately, adequate data is not available for any of the springs currently under consideration.
The schematic diagram that follows shows the relationship of the usage of water from the various sources and potential sources to its current disposition. All water from Harris and Potts Springs travels through the reservoir site. The Town uses much of this water, but a portion is wasted whenever the reservoir is full and overflows. Some water from Cooper Spring is diverted to the water treatment plant when the plant is running and appears to be delivered into Hirst Reservoir when the plant is not running. All water not immediately diverted from Cooper continues downstream and is lost to the Town’s use. There is some uncertainty as to the exact plumbing arrangement at the junction of the ‘Cooper to Hirst’ and ‘Hirst to WTP’ pipelines. The Town currently uses none of the water from Copperhead and the suspected Cooper 2 springs.
Accurate records of the amount of water treated are kept at the water treatment plant. In addition, regular readings (approximately one per week) are taken from a meter in the pipeline between Cooper Spring and the water treatment plant (WTP). Irregular readings have also been made of spring flows from Potts, Harris and Cooper Springs.
The circles in Figure 6-1 show the locations of the regular readings. The locations of irregular readings are shown below by flat ovals.
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Figure 6-1 Mountain Resource Water Measurement Schematic
Potts Harris Copperhead Cooper Cooper 2
1 2 3 4 5 6
Hirst Reservoir
8 9
7
Used by Town not used not used North Fork Catoctin Creek
Locations of regular measurements Legend: Locations of irregular measurements
As can be seen in Figure 6-1, only the segments labeled 4 and 7 are regularly metered. These segments represent the portion of the resource that is currently being used by the Town. For the purposes of this analysis, the most critical data is the portion of the resource that remains unused by the Town. The flow rates in segment 8 and either segment 5 or 9 are the most critical pieces of information. These represent water quantities not in current use. Some information on the flow in segment 8 has been estimated from water levels in Hirst Reservoir, but only a minimal amount of data was available.
The importance of flows in segments 3, 5, 6, and 9 is a function of the potential future location of any efforts to intercept flows from springs in the North Fork Catoctin Creek basin. This aspect is discussed below under “Capturing the Resource”.
A very limited amount of information on the spring flows is available for the locations marked with ovals. The most significant data was located in the Virginia Department of Health files in Culpeper, where information was recovered from correspondence with the Town dated July 7, 1976, September 2, 1976 and October 7, 1980. In addition Town staff has collected some information within the past two years. All of the information available for these locations is shown in Appendix C, Table C-1.
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Ninety one percent of the measurements made at Cooper Spring in 1998 and 1999 indicated no overflow from the spring pool. This indicates that the water plant was using 100% of the flow at those times. Since spring flow is equal to spring usage for a large proportion of the measurements, an alternate method of estimating flow from Cooper Spring is to assume that these two values are equal. Under this assumption, the average Cooper Spring flow is 0.71 MGD.
The known spring flow data is displayed graphically in a series of charts. Figure A-4 in Appendix A shows the published spring flow data from 1976. Figure A-5 in Appendix A shows data from 1980, and Figure A-6 in Appendix A shows data collected during 1998 and 1999. The data is summarized by spring and by collection period in Figure 6-2.
Figure 6-2 Recorded Spring Flow Summary Purcellville - Mountain Resource
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0 1976 1980 1998-1999
Potts Harris Cooper
While this may seem to some to be a significant amount of data, it is well below the minimum amount of data that would be required to analyze these flows. Any data that varies with climatic conditions must be collected on a very regular and systematic basis to be of value for a water resource analysis. This is because the individual data points are not as significant as the long-term statistical compilation. To be of value, the data must be collected over a wide variety of climatic conditions. The data must not only represent this variety of conditions, but the data must reflect the relative frequency and
\Water Resource Study.doc 51 3/14/00 proportions of those conditions. To accomplish this, data must be collected consistently.
While a statistical analysis can be conducted on these random data points, the results do not necessarily represent the variety of climatic conditions that will indicate the performance of a proposed facility. The data that was collected does give some general insight into the flow rates to be expected from these springs, but there is no way to accurately determine a long term yield from this data. As directed by the Town, the minimal available data was used to proceed with this review, but results should be used only on a very preliminary level.
There is a strong data source represented by thirty years of daily data at the stream gauging station at Taylorstown on Catoctin Creek. An attempt was made to correlate the minimal known data points with this consistent record. If successful, this would have allowed the consistent Taylorstown data to be used to fill in the gaps at the desired data locations. Initial results seemed to indicate some relationships within specific data groups. Within the 1976 data and within the 1980 data, some apparent correlation initially appeared. However, when the correlations were checked across the three time frames of available data, the relationships failed. This further confirmed the need for a longer-term database.
C. Capturing the Resource
The same schematic drawing of the Mountain Resource is presented in Figure 6-3 below indicating potential locations to capture the Mountain Resource. These locations are also presented to scale in Map 2 on the following page.
Figure 6-3 Impoundment Location Schematic
Potts Harris Copperhead Cooper Cooper 2
C
Hirst Reservoir A
D
B E
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Insert Map 2 here
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These potential resource capture scenarios include the following:
Site A - raise the existing Hirst Dam Site B - a new dam immediately downstream of the existing Hirst Dam Site C - a new dam 0.4 mile downstream of Cooper Spring Site D - a new dam 0.6 mile downstream of Cooper Spring Site E - a new dam 0.7 mile downstream of Cooper Spring
The quantity of water available to the Town is not only a function of the spring flows, which have been discussed above, but also a function of the location of the capture facility. As the theoretical location of a capture facility is moved downstream, two factors change that influence available yield. These are the natural drainage area of the facility and the configuration of the facility.
As the natural drainage area increases, the flow of water to the facility increases from two sources. First the runoff from rainfall will be increased due to the larger size of the natural watershed. Second, the number of unidentifiable small springs or seeps that feed the facility will also increase as the location is shifted downstream.
The configuration of the capture facility will also impact the yield available to the Town. A reservoir is often thought of as being most significantly influenced by the dam structure that retards the flow of water. However, it is really the size of the pool where the water is stored that is the most significant factor. There are locations within the Town’s Mountain Resource area where a dam could be constructed. However, these locations do not necessarily have a suitable storage volume upstream to retain a sufficient quantity of water to make construction of an embankment worthwhile. Most of the sites on Town property have steep upstream channel slopes that severely limit storage volumes behind an embankment.
D. Other Issues
Two other issues that affect the resource capture location are property ownership and the location relative to the existing Purcellville facilities. Property ownership is addressed in the discussion that follows. The location relative to existing Purcellville facilities could be an issue for certain locations. However, since all of the locations discussed as part of the Mountain Resource option are relatively close to existing Purcellville facilities, this factor will not weigh heavily in a decision among to these options.
E. Individual Sources
Harris and Potts Springs
A very significant component of flows from these springs is already captured. The low potential additional yields for impoundments at sites A and B reflect this. Under the
\Water Resource Study.doc 54 3/14/00 scenarios where a flow-by might be required if a new structure is built, very little additional yield could be expected from these options. This is because serving the flow- by requirement would offset most of the additional yield gained by the larger storage volume. In other words, we could store more water, but we would be forced to release most of it. Expansion of storage at Hirst Reservoir would best be accomplished through the construction of a new impoundment just downstream of the existing dam. This would slightly expand the surface drainage area to the facility and would add to the storage capacity at this location with a minimum of interference to existing operations.
Cooper Spring
For longer-term development of this resource, an impoundment on the stream some distance downstream of the spring site would be preferable. Three locations have been suggested for this impoundment. The first, labeled Site C, is as far downstream from the spring as is practicable while still remaining on Town property. This rather small impoundment would only yield between zero and 0.12 MGD. The only real advantage of this location is that it is located on Town property. Other locations for potential impoundment of Cooper Spring water are discussed below.
Copperhead Spring
If this source is to be tapped, it would probably be most beneficial to capture its flow in conjunction with that from Cooper Spring at the location where the two flows join. A location for an impoundment to capture these flows has been designated Site D on Map 2.
Cooper 2 Spring
To capture flows from Cooper 2 Spring, the preferred location has been designated as Site E on Map 2. Site E is potentially the most useful of the Mountain Resource impoundment possibilities. Its primary disadvantage is that it is not located on Town property. A dam on this site would be located at the point where the current access road to Cooper Spring diverges from the ‘Private Driveway’ just past the end of state maintenance of Spring Crossing Road.
F. Potential Resource Development Locations
Five locations are presented as potential sites to develop the Mountain Resource. These are shown on Map 2 and are labeled as sites A through E. The locations of these interception points will influence the quantity of water available. As indicated in Figure 6-3, interception at Site C will capture water from Cooper Spring only. Interception at Site D would capture water from Cooper and Copperhead Springs. Interception at Site E would capture flows from Cooper, Cooper 2 and Copperhead Springs. Interception at Site A or B would increase the capture of Harris and Potts. Any of these alternatives could be supplemented by diversions of spring flows.
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The three locations indicated for capture of the North Fork Catoctin Creek springs (Sites C, D and E) were located based on several factors. These factors include water availability, terrain suitable for an impoundment, and property ownership. Site E is the best in terms of water availability and terrain. A reservoir at this location could be of a size comparable to the existing Hirst Reservoir. Unfortunately, it is not on property owned by the Town. A facility at Site D would capture less water and the terrain is only suitable for a smaller reservoir. This site is also not on Town owned property, but its location is less intrusive on existing land use in the area. Site C would be suitable for a diversion structure, but the terrain is not suitable for anything but a very small reservoir. This site is located on Town property.
The size of a new impoundment at each location would have an impact on the project cost and water yield at each location. In order to simplify the comparison of the various alternatives, a consistent size structure was selected for the four sites (B, C, D and E) where a new dam might be constructed. This height was set at 30 feet. Site A deviates from this convention because it involves raising an existing dam instead of building a new structure.
Site A
An increase in the height of the existing Hirst Dam was considered. This would not directly bring any additional water sources into the Town’s control, but would provide a larger storage volume to capture and store Harris and Potts Springs flows. Two alternative configurations were considered at this site. A 15-foot increase in the existing dam height was designated as A+15, and a 20-foot increase in the existing dam height was designated A+20.
It would likely not be possible to achieve this increase without obtaining additional property from the adjacent property owner downstream of the existing dam embankment.
The ranges of increase in yield figures due to these options, as published in the Table 6-5 above, is due to the possibility of DEQ using the action of increasing the dam height as a catalyst to institute a minimum flow-by requirement. If such a requirement were instituted by DEQ, almost all gain in yield would be lost due to the new requirement. Without the institution of a flow-by requirement, 0.16 MGD additional yield is all that could be expected based on calculations considering only the watershed area.
If yield calculations were allowed to include spring flows, an additional 0.17 MGD might be added. This addition could also potentially be achieved without building anything at all. Merely modifying the calculation method could add this yield volume.
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Site B
A new dam downstream of the existing Hirst Dam has been a part of water resource development considerations for many years. This site would add a minimal additional drainage area relative to that currently feeding the existing Hirst Reservoir.
As discussed above, if DEQ used the occasion of new dam construction to institute a flow-by requirement, no additional yield would be gained by this modification. Without a flow-by requirement, only 0.12 MGD would be added with this option. As discussed earlier, as much as 0.17 MGD additional yield might be added by redefining the calculation method.
Site C
A new structure downstream of Cooper Spring is one of the concepts proposed by the Town in their directive to ECI on August 8, 1999. Sites immediately downstream of Cooper Spring are generally unfavorable for impoundment construction due to the steepness of the stream channel. A steeply sloping channel upstream of a dam will result in the bottom of the pool very quickly rising to the dam crest elevation as one moves upstream from the dam location. This results in very little storage volume relative to dam height. The channel is so steep immediately downstream of Cooper Spring that a 30-foot high dam there would create almost no pool. See Figure 6-4 below.
Figure 6-4 Impact of Channel Slope on Reservoir Volume
larger pool volume
Dam on Flatly Sloping Stream
smaller pool volume
Same Dam on Steeply Sloping Stream
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As can be discerned from the above diagrams, pool volume is inversely proportional to stream channel slope. Due to this aspect of the terrain immediately downstream of Cooper Spring, a dam at that location was not evaluated.
Moving downstream of Cooper, the channel slope generally becomes flatter as distance from the spring increases. The first site considered, Site C, was selected because it is as far downstream as is possible while still remaining on existing Town-owned property. Unfortunately, the stream channel is still quite steep at this location. The resulting pool volume is therefore small, and the potential yield to be gained by a reservoir at this location is low.
Site E
Site E is the best topographical location for a dam embankment in the Mountain Resource vicinity. A 30-foot high dam at this location produces the largest pool volume of any of the sites considered. In addition, to gauge the impact of alternate size structures on the results of these analyses, a 35-foot high alternate was also considered at this location. This variation is designated as E+5.
Hydrologically, this site is also one of the best because it captures flows from Cooper Spring, Cooper 2, and Copperhead Spring, along with a relatively large watershed. While individually these new springs are not especially significant, together they represent the best potential to gain new yield from the Mountain Resource. With the exception of those Copper flows already being piped to the water treatment plant, all of these are new captures which bring the Town access to flows not currently available.
The most difficult aspect of this site is the existing land uses. The dam location is on two properties that are currently large lot residential properties. The dam structure itself would not impact significant structures, and with rerouting of a driveway, the project could work from a physical perspective. The major impediment will be the reaction of property owners to such an arrangement. It is possible that some owners may be inclined to appreciate a large body of water adjacent to their residence and reactions could potentially be favorable. This will depend on the personal perceptions of the affected property owners. Condemnation would, of course, also be a possibility.
Site D
Site D was selected as a compromise between Sites C and E. Selection of this site was an attempt to gain much of the larger watershed avalable at Site E, while minimizing the impact on outside property owners. The site captures more drainage area than Site C and adds Copperhead Spring to the capture domain. It is unfortunately not entirely on Town property, but crosses the property line between Town ownership and one large lot residential property.
Hydrologically there is considerable loss relative to Site E because Cooper 2 Spring and a considerable area of watershed drainage are lost from the capture domain. There is
\Water Resource Study.doc 58 3/14/00 also a loss relative to Site E topographically because the pool volume created is considerable less than that available at Site E.
The resulting gains in yield are unfortunately closer to the values previously considered at Site C.
G. Yield
To truly assess the ability of these sources to meet the Town’s future demands, a systematic program of determining flows from these springs must be initiated. Development of the magnitude being contemplated by the Town of Purcellville must be supported by factual information, not guesses.
Based on the minimal data available, Table 6-4 has been prepared to indicate potential yield for various reservoir sites. Due to the uncertainty of the data, the yield is presented in three columns. Columns 4 and 5 represent potential yield due to rainfall upstream of the impoundment location. Two alternate results are presented because the Virginia Department of Environmental Quality (DEQ) may dictate that a certain proportion of the natural flow be allowed to pass-by a new reservoir location. However, DEQ has not established what that proportion would be. Since the magnitude of that flow-by requirement would impact reservoir yield, two results are presented with and without that requirement.
Column 6 represents the potential yield at the site due to springs located upstream of the capture location. This information is presented separately, because the accuracy of this portion of the total yield is significantly less than the yield calculated based on rainfall.
A theoretical total yield could be calculated by summing columns 4 and 6 or by summing columns 5 and 6. However, the Virginia Department of Health (VDH) has previously criticized this method of calculating total yield. VDH criticism is based on a contention that the theoretical yield based on watershed (cols. 4 or 5) already includes a appropriate relative proportion of spring flows (col. 6). Thus they argue that column 6 must be ignored. While their perspective has some validity, their assertions can be argued based on the small watersheds and therefore larger relative proportion of spring flows at the sites being considered.
Based on VDH’s position, only column 4 alone or column 5 alone can be assumed to represent yield from these potential reservoir locations. Column 7 indicates what quantity of the yield from each location is already in use by the Town. The portion of those yields already being used must be subtracted to determine the additional yield that would result from any new construction.
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Table 6-4 Yield Calculation Variations
1 2 3 4 5 6 7 Site Drainage Reservoir Watershed Watershed Estimated Current Area Vol. Yield Yield Spring Yield Use (Sq. Mi.) (Ac. Ft.) (MGD) 30% flow-by (MGD) (MGD) Hirst 1.10 120 0.24 0.15 0.17 0.14 A+15 1.10 217 0.34 0.20 0.17 0.14 A+20 1.10 312 0.40 0.26 0.17 0.14 B 1.32 199 0.36 0.21 0.17 0.14 C 0.48 24.2 0.06 0.03 0.14 0.08 D 1.29 38.3 0.11 0.06 0.14 0.08 E 1.73 111 0.27 0.16 0.14+ 0.08 E+5 1.73 186 0.38 0.24 0.14+ 0.08
With all of these uncertainties combined the potential new yield from the five proposed sites are summarized in Table 6-5.
Table 6-5 Additional Yield Ranges
Range of Potential Site ID Site Location Additional Yields
A+15 Raise Existing Hirst Dam 15 feet 0 to 0.10 A+20 Raise Existing Hirst Dam 20 feet 0.02 to 0.16 B New 30’ Dam Downstream of Hirst 0 to 0.12 C New 30’ Dam Downstream of Cooper on Town Property 0 to 0.12 D New 30’ Dam Downstream of Cooper Partially on Town Prop. 0 to 0.17 E New 30’ Dam Downstream of Cooper Off Town Property 0.08 to 0.33+ E+5 New 35’ Dam Downstream of Cooper Off Town Property 0.16 to 0.45+
In addition, the uncertainty of the numerical values in column 6 is still not captured in the ranges presented in Table 6-5.
Evaporation would have some impact on the availability of water from a reservoir. Calculations indicate that each of the proposed reservoir sites would experience the following evaporation rates during the warmer months of the year.
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Table 6-6 Potential Evaporation Rates
Evaporation Rates During Warmer Months Annual Average Rate Site (MGD) (MGD)
A 0.04 0.03 B 0.02 0.01 C 0.00 0.00 D 0.01 0.01 E 0.03 0.02
As can be seen from the values in Table 6-6, evaporation will have a small impact on reservoir yield, although traditionally these estimates are not included in yield calculations.
H. Pipeline from Cooper Spring
As mentioned in the introduction to this report, the hydraulics dictate that water to the treatment plant tends to come from Cooper Spring first before water is drawn from Hirst. This is true up to 350 gpm, the highest value tested. Since this is well above the assumed average flow of Cooper Spring, little value would accrue from an upgrade to this pipeline. It already has well above the capacity that is used on a day to day basis.
Because of this characteristic, the drainage area of Cooper Spring might also be considered in calculating the yields of existing Hirst Reservoir or potential reservoirs at Sites A or B. An interception of the Copperhead Spring basin could add to the potential watershed of reservoirs at the Hirst site. Table 6-7 shows the potential impact of a pipeline to trap the Cooper and Copperhead basins on the additional yields that are presented in Table 6-5.
Table 6-7 Impact of Cooper/Copperhead Drainage Areas on Potential Yields
SiteRange of Potential Additional Yields
A+20 (from Table 6-5) 0.02 to 0.16 A+20 (with Cooper/Copperhead pipeline) 0.06 to 0.28 B (from Table 6-5) 0 to 0.12 B (with Cooper/Copperhead pipeline) 0.02 to 0.17
The size of a pipeline between the Springs and the Reservoir would have to ensure that peak flows could be accommodated.
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I. Costs of Development
In order to standardize the comparison between the various options, a standard height of 30 feet was selected for each structure. Once the optimal location was determined, an alternate height was selected at that location to focus on size alternatives.
The cost of developing the Mountain Resource has one overwhelming component. That is the cost of the impounding structure. The size and therefore cost of each structure is a function of the shape of the valley cross section at each location. While each of the compared structures has the same height (except for Option A, which is the modification of an existing structure), the cost of the structures varies considerably.
The other two significant cost components are the length of piping required to connect the various locations to the existing transmission system and the need for pumping from some of the locations due to lower elevations.
The cost estimates for each location are summarized in Table 6-8.
Table 6-8 Mountain Resource Summary
ID Cost Additional Dollars Additional Dollars Town Yield per Gallon Yield per Property Median Maximum Gallon Ownership Value Value A+15 $4,635,000 0.05 92.7 0.10 46.4 Partial A+20 $6,520,000 0.09 72.4 0.16 40.7 Partial B $1,920,000 0.06 32.0 0.17 11.3 No C $1,280,000 0.06 21.3 0.12 10.7 Yes D $1,190,000 0.09 13.2 0.17 7.0 Partial E $1,600,000 0.21 7.6 0.34 4.7 No E+5 $1,940,000 0.31 6.3 0.38 5.1 No
To compare the various options, the results are presented in terms of ‘dollars per gallon’ of water. Due to the uncertainty of the yields that can be expected from these structures, two alternative calculations of dollars per gallon are presented. The first is based on the median calculated yield for the site, and the second is based on the best case scenario for each location.
As can be seen from the figures above, the best means to develop the Mountain Resource from both the hydrologic and financial perspectives is Site E. As has been mentioned earlier, this is really the only feasible location for a reservoir to tap this resource. The relatively high ‘dollar per gallon’ cost for the other options is due primarily to the low yields that can be assured from these sites. The very high value for Site A, raising the existing structure is due to both the relatively low yields which could result from the imposition of a flow-by requirement and the fact that to achieve a higher
\Water Resource Study.doc 62 3/14/00 embankment the width of the existing structure would have to be significantly increased. That greatly increases the volume of earth that would have to be moved to execute this option.
J. Recommendations
The quantities of water available from the Mountain Resource are not sufficient to meet the long-term (2050) needs of Purcellville. If this resource is to be tapped, it must be addressed as a mid-term solution and/or as a component of a more broadly based combination of solutions.
The best means to tap the Mountain Resource is Site E+5. If this strategy is selected, a stream-gauging program should be implemented as soon as possible. The gauge should be located at or near the site most likely to be used to tap the resource. Site E+5 is located at the current crossing of the stream by the access road to Cooper Spring. If this option is to be pursued, Engineering Concepts will make recommendations on the methodology and equipment to begin this data collection effort.
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VII. Sleeter Lake
Sleeter Lake is an impoundment of the North Fork Goose Creek located immediately south of the Town of Round Hill. Oak Hill Properties, LLC of Leesburg, Virginia currently owns the dam and most of the shoreline. The impoundment is not currently used for anything other than a minor amount of recreational activity. The vast majority of the shoreline is undeveloped, and what is developed is used for large-lot residences.
The owners of the lake are interested in participating with an entity that would be willing to develop the lake as a water supply resource. The Virginia State Water Control Board has set the Sleeter Lake safe yield at 1.55 MGD, if a 30% minimum flow-by was required. There is currently no assurance that the minimum flow-by will be set at 30%. However, this number will not be known until more specific plans are presented to the Virginia Department of Environmental Quality (DEQ) and their joint permitting process is executed. Flow-by is calculated as passing the specified percentage of the average flow by the site or passing inflow, which ever is less.
Yield
Calculations performed for this study confirmed the State figures at a safe yield of 2.5 MGD with no flow-by and 1.57 MGD with 30% flow-by. These recent calculations include flows during the drought of 1999 through October 1. On October 1, 1999 the theoretical water level in Sleeter Lake would have begun to recover from the effects of the drought, but the lake would not yet have refilled completely. Therefore, calculations through the 1999 drought can not be assumed to be complete.
The yield figures published in this report would be reduced by three factors which have not been formalized but which would affect the ability of the Town to use this lake as a water resource. These are: 1) a potential flow-by requirement as previously discussed, 2) limitations on the drawdown of Sleeter Lake, and 3) use of water by Oak Hill Properties.
The impact to the water volume in Sleeter Lake by its use as a reservoir would be significant. Figure 7-1 below illustrates this. This chart presents the proportion of the pool volume of Sleeter Lake that would have been full had the lake been in use as a reservoir over the past thirty years. The calculations are based on a daily withdrawal of 1.57 million gallons and an assumption of a 30% flow-by requirement by DEQ.
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Figure 7-1 Theoretical Sleeter Reservoir Storage Volume Fluctuation With 30% Flow-by and 1.57 MGD Yield
1400
1200
1000
800
600 Volume(Ac-Ft)
400
200
0 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 Date
If Oak Hill Properties wants to reduce this impact on water surface elevation and thus minimize the aesthetic impact on the lake, the resulting effect on reservoir yield calculations could be significant. If the use of Sleeter Lake was limited to one half of the storage volume, the safe yield with no flow-by would be reduced from 2.50 to 1.59 MGD and the safe yield with 30% flow-by would be reduced from 1.57 to 0.95 MGD.
As part of an agreement to use Sleeter Lake as a water source, Oak Hill Properties has indicated that part of their compensation would include the right to use some of the water for their own needs. The impact of this use could be calculated as a direct reduction of the published yield figure by the amount of use desired by Oak Hill. For purposes of comparing this option to others reviewed in this report, Oak Hill will be assumed to require 50% of the yield of this facility. With a) a flow-by requirement, b) a limitation on drawdown to 50% of the lake volume, and c) 50% of the yield diverted to Oak Hill, the net yield to the Town of Purcellville would be reduced to 0.47 MGD.
Water Quality
The quality of the water in Sleeter Lake has been a concern for many years. VDH had at one time indicated that due to the regular use of pesticides on orchards in the lake’s
\Water Resource Study.doc 65 3/14/00 watershed, the lake was unsuitable as a domestic water source. Between 1982 and 1987, the Department’s position began to change. One factor that contributed to this change was the elimination of orchard operations from the watershed.
The last known testing of water quality in Sleeter Lake was performed on January 12, 1987. At that time there were no indications that the water quality was unsuitable as a municipal water supply. The laboratory reports of these test results are presented in the Appendix C of this report.
If these test results are indicative of the current quality of the water, and if no contaminants exist beyond those identified in 1987, then Sleeter Lake water could be treated by conventional techniques and used as a domestic water source.
While watershed conditions have improved over recent years, it should be noted that the increasing development in the Round Hill area could reverse this trend. The shoreline of the reservoir will likely be developed as low density residential.
Cost of Development
The major cost advantage of using Sleeter Lake as a water source is the fact that the impoundment has already been constructed. The most significant cost items are a pipeline to connect this source to the Town’s system and the cost of treating this water.
Since ECI was instructed to minimize efforts on this option, a detailed cost to tap this source was not computed. However, a construction cost in the order of $2 million dollars and a total of near $3 million dollars for project implementation should be anticipated if this option is selected.
Recommendations
While this option has a lot of uncertainty, and would come with a lot of ‘strings attached’, there is a considerable amount of water available from this source. However, if the water resource must be shared with others, then the development costs can also be shared with others. The Town of Purcellville should contact and pursue a relationship with the owners of this facility. Even if immediate use of this resource is not pursued, a continuing dialogue with this property owner may prove to be valuable in the long term.
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VIII. Summary
Water Quantity
The Town of Purcellville operates a municipal water supply system that consists of three mountain springs, the J.T. Hirst Reservoir that captures the spring flows, a water treatment plant that treats water from the Hirst Reservoir collection system, and three operating wells.
Development pressures in the area are causing current usage to increase at a rate that will cause current capacity to be exceeded by 2006. Forecasts for the next 50 years indicate that future water demands will approach four times the current system production capacity. The most important usage rates are summarized in the table below.
Table 8-1 Key Purcellville Water Usage Values
MGD Hirst Reservoir - current safe yield (VDH) 0.30 Existing Wells - capacity (Town) 0.17 Town Total Capacity 0.54
Existing Treatment Plant Capacity 0.60
Current Usage 0.37 Current Usage Growth Rate 0.028 per year
Forecast 2050 demand 1.8 to 2.0
Primary Alternatives For Additional Water
This Water Resources Report has reviewed options for obtaining additional sources of water to meet increasing demands. None of the options alone will achieve the forecast demands for the next 50 years. The three best options are summarized in the Table 8-2 below.
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Table 8-2 Purcellville Water Resources - Best Options
MGD # years Site E+5 0.31 11 Wells 0.42 15 Sleeter Lake 0.47 17
Note: The heading ’# years’ refers to the number of years of additional supply potentially available at the current growth rate. Note: The uncertainties in the additional water supply numbers published here are discussed in the report. All potential yield figures in this table are subject to significant variations.
No single alternative investigated will meet Purcellville’s long-term water needs. In fact, these top three options together may not be sufficient to meet projected demands in 2050.
Methods To Increase Yield From These Alternatives
All three of the best options have significant components of uncertainty in their yield analysis. With some manipulation, the potential yield figures for all three of these options might be increased.
Site E+5. The potential yield figures for this option might be increased by: a) minimizing flow-by requirements set by DEQ, and b) instituting better data collection procedures to reduce the unknowns in the yield computations.
Wells. Well development efforts have the most uncertainty and potential variability of all the reviewed alternatives. This must be resolved by field test drilling. Well system capacity is based on VDH requirements which result in operations of 13.3 hours per day. This operating period could be increased once a well is in service, but full 24 hour per day operation of most wells is usually not recommended.
Sleeter Lake. Sleeter Lake has much more water than the figures published above. The primary means to obtain additional water from this source would be to convince Oak Hill Properties to allow Purcellville greater access to the resource.
Costs of Development
Since nine alternatives were evaluated for consideration in this report, detailed cost estimates were not prepared for all alternatives. For the purposes of comparison of options, all cost estimates are compared to the potential yields for that option and a resulting ‘cost per gallon’ value was computed. As stressed throughout this report, all potential yield values have components of uncertainty.
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Table 8-3 Cost Vs. Yield Comparison
Median Value Maximum Value Option Cost Additional Yield $ per Gallon Additional Yield $ per Gallon A+15 $5,100,000 0.05 102.0 0.10 51.0 A+20 $7,100,000 0.09 78.9 0.16 44.4 B $2,100,000 0.06 35.0 0.12 17.5 C $1,400,000 0.06 23.3 0.12 11.7 D $1,300,000 0.09 14.4 0.17 7.6 E $1,800,000 0.21 8.6 0.34 5.3 E+5 $2,000,000 0.31 6.5 0.45 4.4 Sleeter $3,700,000 0.95 3.9 Wells $1,000,000 0.42 2.4
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IX. Conclusions
Among the options considered in the detail section of this report, no single alternative is likely to achieve the long-term goals of the Town. It will take a combination of options to meet the Town’s water needs.
Wells
Wells are the most cost-effective solution currently available to the Town. They are also the solution with the most uncertainty in their yield. To meet 2050 demands with this option alone, Purcellville would need to successfully develop about 14 production wells with yields in the range of 100,000 gpd. The probability of achieving this goal is questionable, but it may be possible. To keep up with the current growth rate, a new well of approximately 52,000 gpd yield (65 gpm for 13.3 hours per day) would need to be developed every two to three years.
Cooper Reservoir
A reservoir at Cooper Springs would not be an efficient way to capture this resource. As discussed below, a reservoir downstream of the spring could be a component of a wider water resources development program.
New Reservoir
The best potential for developing a new reservoir is at the location designated Site E+5. The potential yield from a 35-foot high structure at this location would be between 0.16 and 0.45 MGD. The cost per gallon of implementing this option is higher than that for wells or Sleeter Lake, but much of the uncertainty of the yield potential from this source can be eliminated through negotiations with DEQ before detailed design or construction begin.
Expanding Hirst Reservoir
An expansion of the Hirst storage facility either by raising the existing dam or constructing a new dam downstream would yield only an estimated 0.13 or 0.17 additional MGD. Costs for this alternative are significantly higher than for the other options.
New Springs
No new independent sources worthy of investment in the general vicinity of the Town’s existing Mountain Resource have been revealed during the development of this study. Some minor sources were discovered that could, in conjunction with a wider resource development effort, contribute to the overall Town supply.
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Piping from Cooper to Hirst
Based on the streamflow records currently available, more substantial piping from Cooper Spring to Hirst would not develop significant additional yield.
Sleeter Lake
This option was not investigated as intensely as the other options. However, Sleeter Lake has the technical capability to deliver significantly more water than the Mountain Resource. The ability of the Town to access all of this resource could be severely limited by the property owner’s desire to share in the use of the resource, and by the property owner’s potential reluctance to allow full drawdown of the reservoir pool.
Strategy and Schedule
Wells are probably the simplest and least costly option for short-term augmentation of existing water supplies. However, wells should not be thought of as the short term option because all options must be pursued concurrently to achieve 2050 objectives. The pursuit of ‘long-term’ goals cannot be postponed because a ‘short-term’ option is available.
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X. Recommendations
Of the options reviewed in this study, three have presented themselves as alternatives that deserve further consideration. However, none of the options appears to be independently sufficient to meet 2050 demands alone. In addition, all of the recommended solutions include some degree of uncertainty.
The following recommendations are therefore designed to work with those current uncertainties and maintain flexibility during the process of developing these resources. This flexibility will keep options open for the Town as issues arise during the pursuit of these alternatives.
None of the recommendations have been designated as ‘long-term’ or ‘short term’ solutions. The combination of the recommendations will likely be required to meet long- term needs. No one option should be selected as a short term solution because postponement of any of these components could allow intervening events to render that postponed component impossible to obtain in future years.
All of the following recommendations should be pursued concurrently. As all these options need to be pursued, no priority ranking has been assigned. They are listed below in alphabetical order.
Data Collection Program. Although the Mountain Resource does not have sufficient quantities of water to meet the long-term needs of the Town, this resource represents a significant contribution to meeting current needs, and can meet some portion of future needs. In order to more fully understand what proportion of future needs that this resource will be able to meet, a rigorous record keeping program should be initiated. This record keeping program should include daily records of the following information.
Daily Records Pool level at Harris Spring Pool level at Potts Spring Pool level of all three sub-impoundments at Hirst Reservoir Pool level at Cooper Spring Meter reading on Cooper Spring pipeline One minute timed flow rate reading on Cooper Spring pipeline
While some activity along this line has been occurring for many years and the level of data collection has increased recently, data collection is a never-ending activity. This should not be viewed as an activity that will occur for only a specific period. These data collection efforts should end only if the Town decides that there will never be a future interest in this resource.
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These actions can be implemented with minimal capital investment. Fixed staff gauges, which already exist in some of the locations, could adequately serve the purpose. The primary investment would be personnel time.
Pursuit of this activity will result in additional information to support further refinement of options.
Preliminary Engineering Report. Begin preliminary engineering of target solution options to determine more detailed engineering impacts of site conditions. Issues to be evaluated would include:
Detailed pipeline routes Detailed treatment processes Dam foundation suitability Dam construction material suitability Hydraulic impacts of new well connections to exiting pipelines Water quality testing of Sleeter Lake Regular evaluations of newly collected data Inundation zones for impoundments Lake bed permeability
Such additional data will be used to enhance cost estimates with project specific detail. Pursuit of this activity will result in additional information to support further refinement of options.
Site E. The potential development of Site E should be discussed with the two involved property owners. Some property owners might be quite receptive to the presence of a body of water on their property, as this can be an attractive amenity. If there is any possibility that this alternative will be implemented, a daily gauge reading at the Spring Crossing Road access to Cooper Spring should be added to the record-keeping program discussed above.
Pursuit of this activity will be a first step toward acquiring this resource and will provide information to further refine the Town’s options.
Sleeter Lake. Initiate and maintain a formal dialogue with Oak Hill Properties, the owners of Sleeter Lake. It is important to have a fuller understanding of their motivations and the compatibility of those motivations with those of the Town.
Pursuit of this activity will be a first step toward acquiring this resource and will provide information to further refine the Town’s options.
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Wells. Begin a groundwater exploration program including:
Detailed geologic mapping Identification of specific properties for test well Surface geophysical surveys Soliciting of bids from qualified well drillers. Development of a source water protection plan Development of a groundwater level and streamflow monitoring program to monitor the long-term affects of the groundwater withdrawal
Pursuit of this activity will be a first step toward acquiring this resource and will provide information to further refine the Town’s options. Positive results will lead to immediate supplements to the Town’s water supply.
Due to ongoing interest in the groundwater resources of Loudoun County, cooperation with the County government and other ongoing studies is imperative.
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