Final Draft\gwmp\vol4_rev\appndx-2aquamine.doc May 20, 1997 Issue Paper: Aquifer Mining And Over-Drafting 1. Introduction And Background 1.1. Purpose and Scope This issue paper examines the identification of and potential responses to Aquifer Mining and Over-drafting. Preventing aquifer mining and over-drafting is necessary to protect ground water and surface water resources from detrimental impacts and assure future water availability. Aquifer mining and over-drafting generally results from altering the land surface, excessive pumping, or a combination of the two, which excessively upsets the natural water balance. Additional information on water balance is available in a separate issue paper. 1.2. Background Aquifer mining and over-drafting are general terms used to denote the condition caused by extracting more water from an aquifer than is being recharged. Over-drafting occurs whenever pumping exceeds the Safe Sustainable Yield (SSY) of an aquifer. When mining and over-drafting activity continues unchecked, the volume of water in the affected aquifer will continue to be reduced until wells go dry and/or the natural outflow from the aquifer is decreased. Natural outflow is discharged into lakes and streams through springs and seeps on the surface of the land and through underwater springs to lakes or seawater. Adequate natural outflow is essential for sustaining stream base flows, maintaining lake levels, and preventing seawater intrusion. Water naturally discharges from aquifers at a rate which is controlled to a large extent by the amount of recharge. In a geological area like Kitsap County, some fresh water flows directly from aquifers to seawater. Well pumping can cause aquifer levels to drop without causing mining to occur. A lowered aquifer water level reduces the differential pressure between the aquifer and the Hood canal or Puget Sound. The reduced differential pressure results in a decreased flow from the aquifer to seawater. When the reduced flow to sea balances the increased extraction through wells, the aquifer water level will stabilize at a new lower level. If extraction reaches too great a rate, a steady lowering of the aquifer water level will occur over time, causing over-drafting, mining, or seawater intrusion. Over-drafting ground water from the shallower aquifers can have an adverse impact on surface waters and wetlands. To determine if mining is occurring, careful monitoring and analysis of the data must be conducted over an extended period of time. The County has many shallow and deep aquifers, some of which may be connected vertically as well as horizontally. The amount of water that can be safely withdrawn from a particular aquifer without causing over-drafting is difficult to predict. Monitoring aquifer water levels and withdrawals is important to prevent over-drafting. Predicting the capacity of a Kitsap County aquifer is difficult and expensive considering existing data and analysis Volume 4 Appendix 2, Issue Papers Aquifer Mining And Over-Drafting (MO) 1 Final Draft\gwmp\vol4_rev\appndx-2aquamine.doc May 20, 1997 capabilities. Changing factors, such as land use modifications which impact recharge rates, complicates the process. The current best means of assessing aquifer overdraft conditions are to record and analyze static water level over a long period of time, correlating ground water withdrawal and precipitation data, and monitoring for seawater intrusion. 1.3. Aquifers An aquifer is a saturated body of geologic materials that can yield usable quantities of ground water to well. Aquifers can store and transmit large quantities of water between the pore space of the aquifer materials. Aquifers have some form of bottom to them, such as bedrock or other impervious or semi-impervious layer, which helps to contain the ground water. Clay or silt or a combination of the two are common aquifer bottoms or aquitards. In a confined aquifer the ground water is trapped between a bottom and a top layer, or aquitard. If an aquifer is only detained by an underlying aquitard, it is called an unconfined aquifer. Aquifers in Kitsap County are normally recharged by rain water which percolates down through the ground. The actual water available from an aquifer for beneficial use on a sustained basis is much less than the total recharge to that aquifer. Part of the recharge water is held in the soil. Part is discharged to streams, lakes, seeps, wet lands or seawater. The remainder enters aquifers. In general, the percentage of recharge going to shallow aquifers is greater than that which reaches the deeper aquifers. 1.4. Water resources It is estimated that over 80% of the water now being used in the County comes from ground water; the rest from surface water supplies. Surface water supplies are taken from both lakes and streams. The largest supply of surface water in the County is the Casad Dam which serves the City of Bremerton. This dam supplies approximately 65% of the water needs of the people living in Bremerton's service area. A precise evaluation of surface water and ground water available for source development is difficult to make. An early study, the Comprehensive Water Study of Kitsap County for Public Utility District No 1, by Ingam, Hill, 1966, noted that of the 426 separate streams on the Kitsap Peninsula, only 12 have drainage areas larger than 10 square miles and that most of the remainder have less than 1 square mile. As noted in the Ingam/Hill study, not all of the 12 larger streams are suitable for development. In any case, the larger streams and rivers in the county have been closed to further appropriation of water (see WAC 173.515). As a result county residents are becoming more and more dependent on ground water. Thousands of home owners are dependent on shallow aquifers for their water supply. A healthy aquifer level is vital to preventing seawater intrusion. The larger public water systems tend to develop their water from deep aquifers via high capacity wells. The County will become even more dependent on ground water in the future, therefore, we must protect individual aquifers from over-drafting. Volume 4 Appendix 2, Issue Papers Aquifer Mining And Over-Drafting (MO) 2 Final Draft\gwmp\vol4_rev\appndx-2aquamine.doc May 20, 1997 1.5. The Safe Sustaining Yield (SSY) of an aquifer The SSY is the amount of ground water that can be safely and continuously withdrawn without unacceptable reductions in aquifer storage. A general estimate of the SSY for the entire County is 16% of the annual precipitation. The actual amount of water available from a particular aquifer could vary considerably from the general SSY percentage estimate for different areas of the county. (See Vol. I, GWMP for details on SSY or Hypothetical Groundwater Yield (HGY)). The size of aquifers, recharge rates, and yields can vary greatly from area to area thus influencing the SSY. SSY is a dynamic value that can be changed by factors such as increases in impermeable surfaces, changes in vegetative cover, and multi-year climate trends. The determination of SSY requires a comprehensive analysis (basin assessment) of an aquifer system. Comprehensive monitoring is recommended by the State Dept. of Ecology (Ecology) (Steve Hirschey personal contact 7/92). When a continuous drop in the static water level of an aquifer occurs over a period of years, the aquifer should be carefully evaluated taking into account rainfall during that period. When over-drafting is indicated, communities should decide what action to take on a cost-benefit basis. Actions can include reduced extraction, enhanced recharge, injection wells, alternate sources, conservation, reuse, etc. 1.6. Recharge Rates and the effect on over-drafting Sustainable yield can be lowered if recharge is modified (reduced) by activities which occur as a result of development. A reduction in recharge rate can trigger over-drafting. Recharge is reduced if land use activities increase surface run-off. Diverting storm water flow to the sea or shifting from septic systems to sewer also reduces recharge. Installing sewers in an area may be necessary in order to prevent ground water contamination, however, a loss of both ground water and base stream flow may result. For example, over 130 square kilometers (50 square miles) of a suburban area on Long Island, New York experienced a water level drop of 3 meters (10 ft.) when the area installed sewers. (Franke 1968, Water In Environmental Planning). See Attachments A for a general explanation of reduced ground water availability as a result of development. 1.7. Problems caused by over-drafting Aquifer mining or over-drafting can result in seawater intrusion. If aquifer levels are lowered sufficiently, seawater intrusion can occur. If intrusion is serious, regulatory agencies such as the Bremerton-Kitsap County Health Department (BKCHD) and the state Department of Health may close an aquifer to further appropriation. Seawater intrusion policies set by Ecology may curtail ground water use by some well owners in order to stabilize seawater intrusion conditions. (Please note Seawater Intrusion Issue Paper for more details.) Ground subsidence can be caused by the removal of excessive amounts of ground water. The amount of subsidence is dependent on the loss of the aquifer water head and the compressibility of the geological materials of the aquifer and confining layers. In some cases Volume 4 Appendix 2, Issue Papers Aquifer Mining And Over-Drafting (MO) 3 Final Draft\gwmp\vol4_rev\appndx-2aquamine.doc May 20, 1997 where over-drafting has occurred, the soils of the geological formation of the aquifer and/or the confining layers have collapsed in on themselves. It is not known how much ground subsidence could occur if aquifers in the County are over-drafted. John Vaccaro of the US Geological Survey has noted that no subsidence problems have occurred in Puget Sound to date.