Page 3 of 5Recharge from the East of GWMA
The subsurface geologic mapping effort also traced the extent of buried basement highlands within GWMA southwards in the eastern portion of Lincoln County (Figure 5). In this part of GWMA, as well as the eastern portion of the State, these buried basement highlands occasionally come to the surface, forming hills projecting above the basalt. These hills, known as steptoes, are like their namesake Steptoe Butte, common in northeastern Lincoln County (in GWMA) as well as western Spokane County and central Whitman County. Where present, these buried hills probably form barriers to groundwater movement from the higher precipitation areas along the Idaho/Washington border into the greater GWMA region.
This relationship holds true for the West Plains area of western Spokane County. Again, these basement highlands, buried beneath the basalt, form buried hills that interrupt the lateral continuity of basalt interflow zones, and the potential movement of ground water eastwards into the GWMA region.
Ground Water Geochemistry, Age, and Recharge Relationships
The water sources that recharge the aquifer system, and the length of time it has taken for water from various sources to arrive at the location of a given well are recorded in the chemistry of groundwater. In particular, different recharge sources (ancient glacial meltwater, irrigation waters, recharge from present-day surface waters such as lakes, rivers, and canals) can be identified using geochemical tracers and groundwater dating methods.
As part of this study to characterize the aquifer system, selected irrigation, municipal and private supply wells, as well as surface water bodies representing potential recharge sources, were sampled and analyzed for a suite of geochemical and isotopic parameters including major and trace element concentrations, stable isotopes, dissolved gases and age tracers (radiocarbon, tritium, atmospheric chlorofluorocarbons or CFCs) in order to elucidate the origins and hydrochemical evolution of the groundwater and, in particular, to evaluate recharge relationships and timescales in the deeper basalt aquifer system. Tritium and CFCs are used to identify and quantify recent recharge components (a few decades to a few years old), whereas radiocarbon is useful for identifying groundwater that is hundreds to tens of thousands of years old. By combining data for age tracers with different characteristic timescales, it is possible to estimate the time elapsed since the water entered the subsurface and also to identify mixtures of older and younger groundwater in production wells. This information was used in conjunction with the geologic model to identify connections to current potential sources of recharge in the aquifer system.
As shown in Figure 6, relatively young groundwater recharge ages (less than 50 years) are generally found for shallow wells open to unconfined sediment aquifers, indicating direct connections to present day recharge sources (canals, seasonally water-filled coulees, creeks and lakes). In samples from some deeper municipal supply wells in basalt which are open over large vertical intervals, age tracers indicate the produced groundwater is a mixture of old (thousands of years) and young water entering from different flow zones with different connections to present day recharge. This data is used in combination with static water level trends, to evaluate the sustainability of current water extraction rates on a local and regional scale. Radiocarbon and stable isotope data, and the absence of detectable tritium and CFCs, indicate that groundwater produced from deep irrigation wells in the Odessa area completed in the lower Grande Ronde basalt layers is more than 10,000 years old. This part of the aquifer system has not been recharged since the end of the last ice age when climate conditions were cooler and wetter than present. Some specific examples of these findings are given below.
Royal City – Recharge to Wanapum Aquifers Keeps Up With Demand (Figure 7)
In the Royal City area in western Grant County, between the Frenchman Hills and the Saddle Mountains, municipal supply wells completed in the shallow basalt system (Wanapum) have apparent radiocarbon ages up to 1300 years old and detectable tritium indicating that part of the groundwater is less than 50 years old. This is direct evidence of a mixture of older water probably originating as precipitation on the Frenchman Hills and as well recent recharge from leakage beneath the nearby Frenchman canal. Static levels in these wells are stable, indicating that the recharge from natural and anthropogenic (canal leakage) sources is sufficient to meet municipal water demands in this part of GWMA.
Moses Lake Well 18 – Wanapum Well Recharged by the East Low Canal (Figure 8)
Moses Lake Well 18 is also completed in the Wanapum (Lower Roza and Frenchman Springs Formations) and has historically stable static water levels. The water from this well is less than 50 years old as indicated by radiocarbon, tritium and CFC data, which implies a direct connection to nearby East Low Canal through Roza/Upper Frenchman Springs flow zones.
Moses Lake Well 17 – Upper Grande Ronde Well With Limited Recharge (Figure 9)
Moses Lake Well 17 is a deep well sealed into the Upper Grande Ronde (Sentinel Bluffs). Static water levels in this well are declining. The apparent radiocarbon age for groundwater produced by this well is approximately 6000 years, but contains detectable tritium, indicating a mixture of old and young water. The proportion of recharge from young water component is estimated to be less than 20 % and is too little to offset withdrawals (i.e. 80 % of the water production is derived from storage, therefore water levels are declining). There are no geologic features indicating a direct connection to surface water, therefore the young recharge component likely represents limited vertical leakage through the many uncased wells present in the vicinity of the town of Moses Lake. Current production rates from this well therefore are not sustainable as there is no reliable source of recharge to this part of the aquifer.
Odessa Area Deep Irrigation Wells – No Recharge to Lower Grande Ronde (Figure 9)
Deep irrigation wells sealed into the Lower Grande Ronde aquifer system in the Odessa subarea approximately 20 miles east of Moses Lake have declining static water levels, apparent radiocarbon ages ranging from 10,000 to 20,000 years and generally no detectable tritium or CFCs. In one example illustrated in Figure 4, a recently drilled well completed in the Umtanum and Ortley units with an apparent radiocarbon age of 15,800 years shows no geologic connection to any surface water recharge sources. The groundwater produced from this well is essentially non-renewable.
|Last Updated ( Thursday, 19 March 2009 )|