HYDROMETRIC AND GEOCHEMICAL CHARACTERISTICS OF SHALLOW GROUNDWATER, WESTERN NORTH CAROLINA

The North Carolina Department of Natural Resources and Western Carolina University established a watershed research station in Cullowhee, NC to examine groundwater/stream water interaction and recharge in a Southern Appalachian, mountain environment. The watershed has a mixed land use history with gullying at the hillslope-channel interface, indicating past geomorphic instability. The station includes over 40 nested shallow groundwater wells installed on three alluvial and/or colluvial sites. The sites represent different geomorphic settings at watershed scales of 0.4, 4.4 and 62 km². Groundwater levels were measured for one year on average every 17 days. At each well, hydraulic conductivity (K) was determined with slug tests. Stream and groundwater were sampled to develop a chemical fingerprint of major ions, nutrients, and hydrogen and oxygen stable isotopes.

Vertical hydraulic gradients show the direction of near-stream groundwater flow is variable, especially at the upstream, colluvial reach. Variance in head levels is generally higher distal from the stream, as the groundwater receives water from surrounding hillslopes. The average K of materials, 1.5 x 10^-4 cm s^-1, was not significantly different (α = 0.05) between the three reaches, though the highest K values (10^-3 cm s^-1) are in alluvial stratigraphic units. Chemical fingerprinting of the water indicates that, on average, stream water and groundwater have a similar chemical composition. For example, differences in average stable isotope concentrations (deuterium and oxygen-18) between ground and surface water is less than the analytical precision of the analyses, and major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺) are similar in both waters as well. Conversely, the pH of groundwater is, on average, significantly lower than stream water. The average specific conductance of groundwater is higher than stream water (54 versus 43 μS cm^-1), but varies widely in groundwater, from 10 - 149 μS cm^-1. Combined, physical and chemical data indicate a complex, stratigraphically controlled shallow groundwater system that is not continuously connected to stream water. Future monitoring equipment in the watershed will permit an increased focus on recharge, hillslope flowpaths, and rainfall response.