Presentations 2016

Evaluating the Effects of Over Pumping and Drought on Water Supply Well Production Capacities and Pumping Costs

University of California, Davis

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Significant decreases in groundwater elevations are generally caused by pumping more from groundwater basins than they can sustainably supply. Such over pumping can be especially pronounced during droughts when surface water supplies are low. The economic impacts caused by over pumping and drought include increased costs for supplying water and lost revenue from inability to meet water demand. In some cases, the combination of decreased groundwater elevation and existing well depth limits well production capacity as wells run dry. Also, decreased groundwater elevation limits well pump operating capacity and increases cost. Water supply wells are constructed to different depths depending upon a variety of factors that include production rate and water quality requirements, local hydrogeology, and projections of future conditions and supply needs. Deeper wells also have been constructed over time as conditions and construction methods have evolved. As a result, the depths of existing water supply wells vary over fairly wide ranges. Overall, shallower wells are more susceptible to impact from decreased groundwater elevations. Likewise, pumps are initially selected and installed in wells based upon specific existing conditions (i.e., desired production rate, standing groundwater elevation and hydraulic performance of the well) and operations are impacted by decreased groundwater elevations.As groundwater elevations decrease, costs may be incurred to move pumps deeper in the wells so the pumps are adequately submerged below pumping water levels. Further elevation decreases may increase production and maintenance costs, as well as decrease production capacity, as pumping levels drop into the screened intervals of wells and cause screen clogging. In more extreme cases, well replacement costs may be incurred as groundwater elevations drop so low that adequate submergence of pumps cannot be maintained and wells become unusable. Additionally, operating points on pump head-capacity curves shift towards lower production rates, efficiencies decrease and operating times to produce required volumes of water increase. All of these effects decrease production capacity and increase operating costs.The potential impacts of decreasing groundwater elevation on well pumping costs were evaluated for a study area located in California’s Central Valley (greater vicinity of Tulare, CA). Well constructions were characterized through statistical analysis of the elevations for the tops and bottoms of screened intervals obtained from well construction logs provided by the California Department of Water Resources (DWR). Groundwater elevation time series were obtained from the DWR Water Data Library. Estimated trends in well capacity loss were developed by determining the fraction of wells over time that had standing water levels (minus an estimated pumping drawdown) below 1) the top of the screened interval and 2) the bottom of the screened interval. Changes in pumping cost were evaluated by considering ranges in lift requirements based on the standing groundwater elevations (minus an estimated pumping drawdown) at different points in time. Trends in pumping costs were developed by calculating lift cost based on standard calculations. This presentation summarizes findings of groundwater well production capacities and pumping costs in the study area under sustained drought conditions.