An Investigation of Coastal Groundwater Discharge and Associated Nutrient Inputs Using Electrical Resistivity, Temperature, and Geochemical Tracer in Pescadero Lagoon, California

Abstract:

Pescadero Lagoon is a complex lagoon system located on the central California coast in San Mateo County. Over the last decade, external stressors such as degraded water quality, restricted circulation, heightened groundwater withdrawals, changes in the fluvial geomorphology that affect surface water runoff, and widespread agriculture in the watershed have impacted the lagoon. The lagoon system is bounded on the marine side by an ephemeral sand berm that is seasonally closed and so hinders open exchange with the ocean. This berm and the Mediterranean-type climate play an important role in the lagoon’s circulation and water quality. The most high-profile and deleterious effect of reduced ocean-lagoon exchange and restricted water circulation is the occurrence of bottom-water low oxygen events that can trigger seasonal fish kills. This project employed a suite of geophysical and geochemical techniques to better understand the role of groundwater on lagoon water and constituent balances. The main objective of this research was to quantify groundwater seepage rates into Pescadero Lagoon across broad spatial and temporal scales using electrical resistivity, temperature, and Radon-222 (²²²Rn) as tracers of groundwater movement. Resulting seepage rate estimates were then used to derive associated nutrient flux estimates, which can be compared to atmospheric and riverine nutrient load estimates to yield more comprehensive nutrient budgets. The groundwater seepage into the lagoon for the time period of March 2013 to February 2014 was relatively low and did not exceed 0.2 m/day. The timing of the sand berm closure, lack of hydrologic connectivity, and lack of freshwater input proved to be crucial limiting factors in the overall health of the ecosystem.