Identifying the Stream Depletion Paradox by Monitoring a Stream’s Response to Aquifer Pumping from Neighboring Wells

Current groundwater models utilize a constant head (Dirichlet) boundary condition which assumes stream stage is fixed and does not experience any drawdown in the event of pumping from an interconnected aquifer despite the presence of stream depletion. Therefore, constant head boundary implies that streams and lakes in a groundwater model behave as an infinite supply of water when aquifer pumping occurs. This study aimed to determine if a stream located in the California Central Coast experiences drawdown and depletion when pumping occurs within an aquifer-stream system. This was achieved by measuring stream stage, aquifer water levels, stream discharge, and the hydraulic conductivity of the subject streambed. Passively collected stream stage and aquifer water level data, actively collected stream discharge data, and in-situ streambed hydraulic conductivity measurements were taken from August 2022 to August 2023 and analyzed using time series analyses and hydraulic conductivity calculation methods. This study confirmed that stream depletion occurred during aquifer pumping and, at low discharge rates, stream stage exhibits observable drawdown in response to aquifer pumping which contradicts the constant head boundary assumption and confirms the existence of the stream depletion paradox in the subject aquifer-stream system. The streambed hydraulic conductivity was found to be relatively high and contained highly conductive gravels and coarse sands implying that the streambed has relatively high storage capacity in the subject stream. This research, subsequent data collection, and improvements to groundwater modeling will allow water managers and planners to sustainably manage local water resources which will be relied upon for generations to come