Quality of Shallow Ground Water in Areas of Recent Residential And
Total Page：16
File Type：pdf, Size：1020Kb
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM Quality of Shallow Ground Water in Areas of Recent Residential and Commercial Development in Salt Lake Valley, Utah, 1999 Great Salt Lake WYOMING Basins Bear Significant Findings Lake study IDAHO • The nitrate concentration in water sampled from 26 of the 30 monitoring wells was higher area UTAH than an assumed background level of 3 mg/L. • At least one of the herbicides, atrazine, prometon, simazine, or tebuthiuron was detected in water sampled from 25 of the 30 monitoring wells. Great Salt Lake Salt • Chloroform, most likely from chlorinated public-supply water that has recharged the Lake shallow aquifer, was detected in water sampled from 27 of the 30 monitoring wells. City • Tetrachloroethene was detected in water sampled from 16 of the 30 monitoring wells with 1 concentration greater than the U.S. Environmental Protection Agency Utah 5 µg/L maximum contaminant level for drinking water. Lake Introduction 111°37'30" Residential and commercial develop- ment of about 80 square miles that pri- 112°15' Jordan marily replaced undeveloped and agricul- Great Salt tural areas occurred in Salt Lake Valley, 80 WASATCH RANGE Lake Utah, from 1963 to 1994. The effects of 40°45' Tailings ponds human activities on the quality of shallow SALT LAKE VALLEY 15 ground water in the recently developed River areas were studied by the U.S. Geological Survey (USGS) as part of the National OQUIRRH MOUNTAINS Water-Quality Assessment (NAWQA) program. The land-use study consisted 215 of 30 monitoring wells installed and sam- pled in 1999 in residential/commercial areas where shallow ground water has the potential to move to a deeper public- supply aquifer. The water samples were analyzed for major ions, nutrients, pesti- Salt Lake County cides, volatile organic compounds 40°30' Area of study shown in figure 2 (VOCs), trace elements, and radon. The occurrence of nitrate, pesticides, and Approximate limit of basin-fill VOCs in water sampled from these wells material in Salt Lake Valley can serve as an indicator of water affected Base from U.S. Geological Survey digital line graph data, 0 3 6 MILES 1:100,000 , 1979 and 1980 by human activities at land surface. This Universal Transverse Mercator projection, Zone 12 0 3 6 KILOMETERS report describes the nitrate, pesticide, and Figure 1. Location of residential/commercial land-use study area in Salt Lake Valley, Utah. VOC data collected during the study. dan River, which flows north along the The population in Salt Lake County Description of Study Area axis of the valley and discharges into in 1999 was about 850,000 (U.S. Census Salt Lake Valley covers almost 500 Great Salt Lake. The climate in Salt Lake Bureau, written commun., 2000) and is square miles and contains the Salt Lake Valley is semiarid, with a normal precip- growing rapidly. The population almost City metropolitan area (fig. 1). It is bound itation rate of about 12 to 20 inches per doubled between 1963 and 1994, corre- on the east by the Wasatch Range and on year. Lawns and gardens in the valley sponding to areas with recent residential the west by the Oquirrh Mountains. require irrigation to supplement precipi- and commercial development in the valley Mountain streams discharge into the Jor- tation during the growing season. (fig. 2). The cities of Sandy and West U.S. Department of the Interior USGS Fact Sheet 106–00 U.S. Geological Survey July 2000 South Salt Lake EXPLANATION West Jordan have experienced substantial Valley Approximate limit of basin- growth. An adequate supply of water that City fill material Discharge area is suitable for domestic use is one of the Taylorsville most important factors in sustaining the Primary recharge area current population and in allowing for Murray Secondary recharge area continued economic growth in Salt Lake Recent residential/com- mercial land use Valley. Area that meets recent Cottonwood West Jordan residential/commercial Ground Water in Salt Lake Heights land-use study criteria 2 Valley Nitrate A generalized model of the saturated Nitrate concentration, in milligrams per liter basin-fill material in Salt Lake Valley South Jordan Less than 0.05 consists of a relatively deep unconfined 0.05– 3.0 aquifer near the mountain fronts that Sandy 3.0 – 5.0 5.0 – 10.0 becomes confined toward the center of 10.0 – 13.3 the valley by interbedded, discontinuous layers of silt and clay (fig. 3). Overlying Riverton Draper Nitrate this confined aquifer is a shallow, gener- ally unconfined aquifer. The primary South Salt Lake West recharge area for the deeper aquifers is Valley Atrazine City near the mountain fronts where there are Atrazine concentration, in no substantial layers of fine-grained ma- Taylorsville micrograms per liter Less than 0.001 terial to impede movement of water. Murray 0.001–0.020 Downward leakage of water from the 0.020–0.100 shallow aquifer to the deeper confined 0.100–1.00 1.00 –1.58 aquifer is possible where a downward Cottonwood gradient exists and confining layers are West Jordan Heights thin, absent, or discontinuous. These con- ditions can exist in the secondary recharge area. The shallow, generally unconfined South Jordan aquifer is susceptible to contamination Sandy from activities related to land use because of its proximity to land surface. Water from this aquifer is not currently used for Riverton drinking. The deeper unconfined aquifer Draper Atrazine also is vulnerable because of a lack of South Salt Lake Tetrachloroethene confining layers that can impede the West Tetrachloroethene concentration, downward movement of contaminated Valley in micrograms per liter City ground water. Water quality in the deeper Less than 0.1 Taylorsville 0.008– 0.1 (estimated values) confined aquifer can be degraded by the 0.1 – 1.0 secondary recharge of contaminated water Murray 1.0 – 5.0 from the shallow and deeper unconfined 5.0 – 7.8 aquifers. The deeper unconfined and con- fined aquifers in Salt Lake Valley are Cottonwood West Jordan Heights used extensively for public supply. More than half of the population in the valley receives ground water for household use. 0 2 4 MILES Volatile organic compounds have been South Jordan 0 2 4 KILOMETERS recently detected in water pumped from public-supply wells completed in deeper Sandy N aquifers. These wells are primarily in urban/residential areas within the valley (Thiros, 2000). Tetrachloroethene was Riverton Draper Tetrachloroethene detected in water from seven public- supply wells in Salt Lake and Davis Coun- Figure 2. Distribution of nitrate, atrazine, and tetrachloroethene concentration in water sampled ties. Contamination of drinking-water from monitoring wells in Salt Lake Valley, Utah. supplies from VOCs and pesticides is a human health concern because many are EXPLANATION toxic and are known or suspected human Direction of ground-water movement carcinogens (U.S. Environmental Protec- Primary tion Agency, 1996). These compounds recharge are difficult and expensive to remove area Secondary from ground water. Additional data and recharge interpretation are needed to determine area Discharge the occurrence, distribution, and sources area of VOCs in the shallow aquifer and the Perched deeper drinking-water supply aquifers in Deeper water unconfined Salt Lake Valley. aquifer Deeper Site Selection and Well confined Installation aquifer Shallow Study sites were selected with a com- unconfined aquifer puterized, stratified random selection Water level in shallow process (Scott, 1990). It identified 41 unconfined sites in Salt Lake Valley that met the study aquifer criteria: (1) location in residential and Fine-grained Consolidated material rock commercial areas developed from 1963 to 1994; (2) 75 percent of a 500- meter Water level in Shallow Basin-fill material of deeper aquifers confining layer Tertiary age circular buffer around the site contains targeted land use; (3) a downward gradient Figure 3. Generalized block diagram showing the basin-fill ground-water flow system, exists between the shallow and deeper Great Salt Lake Basins Study unit. (Modified from Hely and others, 1971.) aquifers; and (4) a minimum distance of 1 kilometer exists between each site field (Koterba and others, 1995) and in unconfined aquifers, it was less than 1 (Squillace and Price, 1996). Areas devel- the laboratory to evaluate and ensure the mg/L (110 analyses). oped after 1994 were not included in this reliability of the data. Nitrate concentration in water sampled study because of the time necessary for Chemicals Detected in Shallow from 26 of the 30 monitoring wells (86.7 new construction to affect the ground- percent) was higher than an assumed water system. Areas developed before Ground Water background level of 3 mg/L, indicating 1963, such as downtown Salt Lake City, Nitrate a possible human influence. Concentra- were also excluded because of a greater tions ranged from less than 0.05 mg/L to Although nitrate as nitrogen can occur potential for the land use to have changed 13.3 mg/L (fig. 2). The median nitrate naturally in ground water, elevated con- with time. concentration for the 30 samples was 6.8 centrations in urban areas are typically Monitoring wells were installed at 30 mg/L. caused by human activities. Some of the of the 41 sites according to NAWQA potential sources of nitrate in ground The U.S. Environmental Protection protocols (Lapham and others, 1995) (fig. water include nitrate leaching from areas Agency’s (EPA) maximum contaminant 2). Depth of monitoring wells ranged where manure has been applied, leaking level (MCL) for nitrate in drinking water from 23 to 153 feet, and the wells were or improperly functioning septic systems (10 mg/L) was exceeded in water from 3 completed with a 10-foot length of screen. and sewer pipes, and nitrogen-based fer- of the 30 wells (10 percent).