UC Davis

Groundwater flow systems are strongly influenced by heterogeneity and anisotropy of hydraulic conductivity (K). Particularly in stratified clastic sedimentary aquifers, the vertical hydraulic gradient (dh/dz) is often very high, due to the low vertical hydraulic conductivity (Kv) or high anisotropy (Kh/Kv). However, data on the vertical hydraulic gradient to calibrate the anisotropy is seldom available. We investigated the relationship between the variable Kh/Kv and the dh/dz computed by a groundwater flow model to improve the regional parameterization using an extensive data base on 3D distribution of hydraulic head. Although it is commonly assumed that the value of Kh/Kv is 10, our values typically ranged between 10 and 105, because the maximum Kh/Kv was 105 in this regional basin. The simulations used MODFLOW-2000. We found that the high Kh/Kv contributed to high dh/dz, while identifying locations of potentially good vertical connectivity between the aquifer zones, where dh/dz is very low and difficult to measure sufficiently accurately. Sensitivity analysis of dh/dz to recharge and pumping from wells showed that recharge led to change in inflow of groundwater that led to head change, which in turn led to change in dh/dz. While, pumping contributed to aquifer outflow, as change in head drawdown around the influence zone, in turn changed dh/dz. Additionally, dh/dz in a shallower aquifer was more strongly affected by recharge and pumping.