MODELING TIDAL DILUTION OF GROUNDWATER DISCHARGING TO SAN FRANCISCO BAY

Rising tides introduce surface water (sw) into aquifers at coastal margins. Although the net flux of groundwater (gw) is not changed by tidal influence, the concentration of chemicals in the gw that discharges during low tide is reduced by near-shore mixing of sw and gw in the aquifer.

Regulation of chemicals entering SF Bay is based on toxicity to potential aquatic receptors. However, in most cases, the mixing of sw and gw takes place very close to the shoreline and it is difficult to measure the concentration of chemicals at the gw-sw interface. Modeling is useful to estimate an attenuation factor for chemicals in gw discharging to the Bay.

We developed a flow and transport model using MODFLOW and MT3D to evaluate attenuation of chemical concentrations in gw due to dilution associated with tidal mixing in the fill close to the Bay. The model consists of 400 one-foot-long cells. At the upgradient end, head is fixed at an elevation of 4 ft. At the downgradient end, which represents the Bay, head is specified to oscillate diurnally with amplitude of 6 ft about a mean elevation of zero. The boundary conditions produce an average hydraulic gradient of 0.01 toward the Bay. The base of the aquifer is at an elevation of –4 ft, resulting in an average saturated thickness of 4 ft at the Bay margin and 8 ft at the upgradient boundary. For the base case, K is 75 ft/d and Sy is 0.2. An effective porosity and longitudinal dispersivity of 0.2 and 1 ft, respectively are specified. A constant source of dissolved chemicals is located 150 ft from the bay margin. Results are calculated for a 500 day period, which is sufficiently long to see steady concentrations evolve.

The base case model shows a 65% reduction in the average concentration of chemicals in gw before it enters the Bay, and the inland extent of mixing extends only 30 ft into the aquifer from the Bay. Sensitivity analyses, which include varying K and Sy, indicate a range of 45 to 80% for the reduction of average concentration, and a distance from the Bay over which attenuation occurs ranging from 20 to 35 ft. Our tidal dilution modeling produces more conservative results than obtained using the solution of Yim and Mohsen (1992). The latter, which considers only confined conditions and uses a less realistic tidal boundary condition, predicts a 10-fold reduction of concentrations within 50 ft of the Bay.