Issue paper: Aquifer Water Balance

Transcription

1 Issue paper: Aquifer Water Balance 1. Introduction And Background 1.1. Purpose and Scope The population in Kitsap County has grown rapidly in recent years and is expected to increase substantially in the future. Demand for water will increase with population growth. Ground water provides over 80% of the total water supply within Kitsap County. This percentage is expected to increase given the cost and regulatory difficulties in developing surface water resources. Actual estimates of the amount of ground water available are important to determining what monitoring is prudent to evaluate the impact of increased withdrawal on the county's aquifers. This issue paper discusses factors which affect water balance with aquifers Water Balance A water balance is an assessment of the major components of a hydrologic system and includes the interactions between surface water and ground water systems. A water balance assessment provides a general understanding of the magnitude of the recharge and discharge components. It does not provide an accurate assessment of surface water/ground water interactions and quantities, and should not be relied on as the sole tool for ground water management. The components of a simplified water balance equation can be expressed as: Precipitation = Evapotranspiration + Run-off + Recharge The water balance components are described as follows: Precipitation (rainfall) varies dramatically in Kitsap county from less than 30 inches a year in the North portions of the county to more than 70 inches a year in the Southwest. Evapotranspiration is water that is returned to the atmosphere. It consists of the moisture that is transpired by plants and evaporated from land surface and surface water bodies. The evaporative losses are highest during the summer when temperature conditions and plant activity are at a maximum. Run-off (storm water run-off) is the water that collects on the land surface or shallow subsurface and flows quickly to the streams, rivers, and other drainage systems. In Kitsap County, run-off quickly ends up in Puget Sound or Hood Canal. Over land flows are largest during storm events and are less noticeable during periods of light rain. The amount of run-off is controlled in part by local soil Aquifer Water Balance (WB) 1

2 conditions, topography, and vegetation. Vegetative and porous soils tend to absorb storm water. In addition, flat surfaces and retention features slow down rates of over land flow, thereby giving water more time to be absorbed into the soil. Recharge is the portion of precipitation that is infiltrated into the ground past the root zone. Recharge percolates down through the soil until it reaches the water table or an impermeable surface. A large percentage of ground water re-emerges into lakes and streams, and constitutes surface water base flow. The geology of the county is such that impermeable layers cause significant amounts of ground water to move in a horizontal direction toward the sea. Only a small percentage of recharge infiltrates to the deepest aquifers. Under natural conditions, all of the recharge eventually will pass to the sea through rivers, streams or by direct discharge via submarine springs. Water pumped from aquifers interrupts this natural balance. The amount of recharge is directly affected by the amount of run-off and evapotranspiration. If run-off is a large component of the water balance, less water will be available for recharging ground water supplies. Ranges of values for water balance components were estimated for various sections of the County during development of the Ground Water Management Plan (GWMP) (refer to GWMP, Vol. 1, Table II-14). Estimates generally fell in the following ranges: evapotranspiration 30% to 60%, run-off 10% to 30%, and recharge 25% to 50%. The recent County basin assessment project used an ET estimate of 38%. The water balance of an aquifer system is not a fixed condition. It will change seasonally and from year to year. All the components of the water balance can deviate dramatically over time from natural and/or human activities. For example, the long-term average annual precipitation at Hansville may be 27 inches but in any given year the annual total can vary significantly from the long-term average value. Changes in run-off and evapotranspiration may change the water that is available to recharge the ground water supplies. Clear-cutting, for instance, will decrease the evapotranspiration and could temporarily increase run-off depending on slope, soil characteristics, and mitigating action taken. It is important to understand that the water balance components and ground water recharge can be significantly influenced either positively or negatively by human activities. As an example, water from run-off can be retained, treated if necessary, and reintroduced into the ground water system thereby enhancing natural recharge. Development on the other hand can have a negative impact on recharge. Urban development of forest lands will increase impervious surfaces and decrease vegetative cover thus increasing runoff and altering evapotranspiration rates. The percentage of precipitation which recharges ground water is reduced by paving or diverting rainwater away from recharge areas. (Note TR-55 US. Soil Conservation Service, Urban Hydrology for Small Watersheds, the Changing Characteristics of the Water Balance Components) Aquifer Yield Terminology Aquifer Water Balance (WB) 2

3 A number of terms have been coined in an effort to qualitatively address the ground water yield from an aquifer. Hypothetical Groundwater Yield (HGY) was a term developed in GWMP, Vol. 1, 1991 (page II-42, and Table II-15) to describe an approximate range in the amount of ground water that could be potentially developed within areas of the county. HGY was assumed to represent the amount of water (expressed as a percentage of precipitation that effectively recharges ground water) that could be safely withdrawn from the ground water system without resulting in over-drafting. (See the Aquifer Mining and Over-drafting Issue Paper for additional information). This is also the general definition given to Safe Sustainable Yield (SSY). (Note: SSY is used in RCWs (RCW )). Some estimates indicate that as much as 30% to 50% of recharge can be withdrawn without causing an overdraft of ground water resources. Others estimates are far more conservative indicating 10 to 20% recharge. HGY is based on recharge quantities that contribute to shallow aquifer systems. Recharge to deep groundwater systems may be substantially less than for shallow systems because of the occurrence of confining units which direct groundwater toward surface water discharge points. The sustainable yield for deeper aquifers may be only a fraction of the HGY estimates. Please refer to GWMP Vol. I for a full discussion of the development of HGY. An earlier study estimated that Kitsap County would experience ground water depletion (also called over-drafting) between the years 2000 and 2025 (See PRELIMINARY ASSESSMENT OF WATER RESOURCES AND PUBLIC WATER SERVICES ISSUES IN KITSAP COUNTY, completed by Kitsap County Dept. of Community Development of Kitsap County and Kitsap Public Utility District #1, 1986.). This assessment used existing population growth and was based on Safe Deliverable Yield (an earlier variation of HGY) County Wide Water Balance Data analyzed as part of the Kitsap County Basin Assessment and information from GWMP Vol. I provides estimates for several components of the water balance. An evaluation of newly collected precipitation data in combination with existing long term data, resulted in an estimate of 315 billion gallons of rainfall annually. Rainfall either is evaporated, absorbed (and transpired) by plants, recharged to ground water, or diverted to run-off which moves to sea through storm drains, streams, or rivers. The Initial Basin Assessment project estimate of evapotranspiration(et) was 113 billion gallons of rainfall evaporating or being absorbed and transpired via vegetation in Kitsap County on an annual basis. The total annual discharge to sea via storm drains and an estimate of storm water that discharges to sea via streams and rivers is not available. Total storm water run-off estimates from various studies in the region run between 15% and 25% of rainfall or between 47 and 79 billion gallons in Kitsap County. For this analysis, 63 billion gallons per year will be used. Total groundwater recharge based on the precipitation and evaporation estimates would be approximately 140 billion gallons. Several things can happen to the estimated 140 billion gallons of recharge that percolates beneath the root zone : Aquifer Water Balance (WB) 3

4 A large percentage of the water that recharges will resurface through springs and seeps forming the base flow for streams and rivers. Estimates of total base flow for streams and rivers in the county is not available. Diversion of surface water for beneficial use in Kitsap County is not extensive. The only major diversion is the City of Bremerton's Casad Dam which provides 1.8 billion gallons of water per year out of a total surface water use of approximately 2.4 billion gallons per year. The remaining water demand is met with ground water. Total surface water rights and claims in the county amount to 3.3 billion gallons of water per year. Well over fifteen thousand wells extract ground water for various uses. Estimated annual extraction of ground water in Kitsap County is 9.8 billion gallons per year. Total ground water rights and claims for the county is 25.4 billion gallons per year. Total water usage in 1995 is estimated to be approximately 12.2 billion gallons. A portion of the water extracted from the ground (as well as diverted surface water) is returned to the ground by irrigation and septic systems. A substantial amount of water is processed through sewage treatment plants and discharged to sea. Total Sewage Treatment Plant discharge is estimated to be over 4.8 billion gallons per year. The remaining recharge flows to Puget Sound and the Hood Canal where it emerges underwater via submarine springs. The total amount of flow via this path has not been determined. Exhibit 1 provides a graphic presentation of the above estimates. 2. Current Laws, Practices And Procedures 2.1. Water Availability Assessment Of vital interest to ground water administration and growth management is devising a reasonable method to assess the amount of water available for beneficial use without causing a depletion of the resource. (Note: Discussion on Water Balance and Recharge in section G page II-38 Vol. 1 of the GWMP). The Growth Management Act makes local governments responsible for investigating and determining if enough water is available before issuing building permits. Various methods and models are available for estimating water availability, but their accuracy varies considerably depending on the scope of the assessment (i.e. site specific or semi-regional), the quantity and quality of available data, and other factors. Given the complex geologic environment of Kitsap County, the use of comprehensive, sophisticated computer modeling can provide insight, but at considerable cost. 3. Gaps And Problems Aquifer Water Balance (WB) 4

5 3.1. A. Determination of HGY, SSY Estimates Of Aquifer Capability HGY, SSY and other previous methods of determining water availability within Kitsap County provide only rough estimates for broad based planning strategies. More rigorous estimates of water availability will necessitate long-term monitoring of critical water balance components, a more in-depth characterization of aquifer systems, and detailed analysis which, in some cases, may include quantitative computer models Data Collection and Monitoring The Data Collection and Analysis Plan (DCAP) for ground water, July 1989, and Volume 1 of the Kitsap County GWMP, Background Data Collection and Management Issues, identifies seven Kitsap County aquifers which may be showing signs of overdraft. The studies also indicate that certain sections of the county could experience water supply problems early in the next century. These trend assessments are preliminary due to data limitations. Additional monitoring and analysis of county aquifers need to be conducted Seawater Intrusion Because the county is surrounded almost entirely by the Hood Canal and Puget Sound and because rainfall in the northern sections is relatively low, seawater intrusion is a significant threat. Monitoring and analysis for seawater intrusion as a result of increased water withdrawal is currently inadequate. 4. Recommendations And Strategies WB 1. County and city government should gather data on the dynamic nature of the relationship between land use activities and their impact on surface and ground water. WB 2. Kitsap County Public Utility District should develop an educational program for the public, public officials, and the business community regarding the effects of run-off on the water balance and the impact development has on storm water and ground and surface water supplies. WB 3. County and city government should adopt economical and feasible run-off practices which enhance ground and surface water supplies. WB 4. Appropriate government entities should continue to search for policies and practices that are beneficial to enhancing water supplies. WB 5. Kitsap County Public Utility District should develop a comprehensive aquifer monitoring plan that will provide data which can be used to identify over-draft conditions and facilitate corrective action. WB 6. County and city government should develop building codes which enhance retaining and recharging as much run-off as possible. Aquifer Water Balance (WB) 5

6 WB 7. County and city governments in conjunction with Ecology, should investigate the feasibility of collecting and artificially recharging run-off during periods of high precipitation. WB 8. Kitsap County Public Utility District should conduct a comprehensive evaluation of the south and west areas of the county for water supply potential. WB 9. If County and City governments convert significant numbers of households from septic to sewer, consideration should be given to reusing treated waste water in order to compensate for lost recharge from the abandoned septic systems. Aquifer Water Balance (WB) 6

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