Using Dupuit’s equations to build a topographically based and GIS-driven water table model for estimating water table position and transit times

Abstract:

Groundwater transit times and water table position serve as foundational controls of how catchments function; the distribution of each influences the movement of solutes through the landscape, and the subsequent biogeochemistry of the system as a whole. Understanding the fate and transport of soluble pollutants, like nitrate and phosphate (common in most agricultural and silviculture settings) is an important consideration in land management. For land managers who seek to understand the groundwater hydrology of the catchments, the time, resources and expertise needed to model groundwater behavior are consistent barriers. Here we present a 2-D, topographically based, and GIS-driven model of water table position that can be used by resource managers to estimate water table position, providing support for analysis of solute transport. This 2-D model is based on Dupuit’s equations for one-dimensional horizontal flow in a phreatic aquifer between two parallel rivers. Water table elevation along each point on topographically-defined drainage pathway is calculated, and the resulting water table profiles are statistically smoothed. The resulting flow paths and pore velocities are used, along with a detailed set of field-based measurements, to address questions of the fate and transport of nitrate from fertilized pine plantations in three adjacent low-relief, groundwater driven headwater streams that drain the Savannah River Site in the Upper Atlantic Coastal Plain of South Carolina, USA. An observed pulse of nitrate was identified following the clearcut and fertilization of the site, but only at the top of the surficial aquifer. Seven years after the initial increase, this nitrate has not appeared in the riparian groundwater or streamwater but has persisted in the surficial aquifer. This apparent lack of transport from the surface to groundwater and the stream system has led to questions of groundwater ages and transit times at the site. We use the proposed water table model to estimate catchment-wide transit times as a mechanism to evaluate these lingering questions of nitrate transport from the plantations.