Parameter estimation using groundwater age and head data, Cape Cod, Massachusetts

Abstract

A practical inversion procedure is used to parametrize an unconfined aquifer on Cape Cod, Massachusetts, using groundwater age and hydraulic head data. The inversion resulted in estimates of the recharge flux, homogeneous but anisotropic hydraulic conductivity, porosity, boundary hydraulic heads, and the aquifer thickness. The range of estimated values agreed well with independent measurements at the site. By themselves hydraulic heads are sensitive to the ratio of recharge flux to hydraulic conductivity. The age data are sensitive to the ratio of recharge to porosity. Together, age and head data provide a constraint on both boundary values and material properties. However, a sensitivity analysis shows that porosity, hydraulic conductivity, and recharge are all correlated, and thus a unique inverse solution requires an independent constraint on one of these parameters. Simulations also show that groundwater ages at a given point on a streamline are sensitive to the recharge flux at the point where the streamline begins at the water table. This implies that a vertical profile of age data will be sensitive to the upstream recharge flux, suggesting that spatial patterns in recharge can be estimated using vertical profiles from multilevel monitoring wells. Because measurements of groundwater age can be made in many shallow aquifers, this approach may be applicable to other sites.