Note: This is “sub” post that supports other blog entries.
Before embarking on my overly simplistic explanation of Western Washington ground water hydrology, I should disclose, I have a chemical engineering degree and worked as a petroleum reservoir engineer for five years before law school. I have a reasonably good understanding of the physics governing water flow through an aquifer, wells, treatment systems, and piping, but I definitely am not an expert in the local hydrology or aquifers. You should consult such an expert about your particular circumstances.
With that disclaimer, the first principle is that actual amount of precipitation we receive over the course of a year hardly ever equals the 50″ average annual precipitation recorded between 1897 and 2015 (using Olympia airport NOAA data). In 1999, for example, we got a whopping 65″ of precipitation. The very next year we received only 36 inches. The average annual precipitation for those two years was 52″ (close to our long term annual average), but one year was unusually wet and the next unusually dry. The hydrological conditions in your aquifer were likely dramatically different in those two summers and different from conditions that exist in the “average” year.
Second, South Sound watersheds receive a lot more precipitation (about 85% of the annual total) from October 1 through April than during the rest of the year. On average the Olympia airport receives about one-half inch rain for all of July. In November, December, and January, there are many single days that receive that much precipitation
Third, the groundwater reservoir (aquifer) that your water well taps into is probably hydraulically connected to the aquifer that your neighbors are (or will be) pumping from and to the surface water bodies (i.e., streams and lakes) that recharge the aquifer. Those hydraulic connections mean that your groundwater withdrawal will to some extent reduce the amount of water available for your neighbors to pump and/or the water available for aquifer recharge. Correspondingly, your neighbor’s groundwater withdrawals will reduce the amount of water available for you to pump from the aquifer. Equally important the amount of precipitation received in the watershed affects the aquifer recharge and the amount of groundwater available for your well.
These principles result in the observation that local groundwater levels typically peak around March 15th and reaches their lowest level in November although those dates can vary significantly in any year. The seasonal variation in ground water levels is easily seen in measurements taken in wells. For example in published data from water wells located in the Scatter Creek area of South Thurston and North Lewis County, the recorded static water levels typically varied by 15 feet between the wet and dry months of the same water year. That variation caused some water wells (i.e., those with pumps set above the lower static water level) dry up in the summer and fall. This hydraulic variability is not a recent phenomenon; although population growth (i.e., more wells sucking from the same aquifer) exacerbates the effect. Since the 1950’s summertime groundwater shortages have been reported by homeowners in the Scatter Creek area.