Modelling the impact of historical land uses on surface-water quality using groundwater flow and solute-transport models

Groundwater age, and its influence on contemporary water chemistry, needs to be accurately described to quantify the temporally varying impacts of land use on water quality. The time lags between solute inputs at the land surface and impacts on stream chemistry can be an important factor for managing land use in regional watersheds. Our approach uses a modified groundwater flow code to simulate reverse groundwater flow, regional flow and the solute-transport model where a unit concentration of a conservative solute serves as a proxy for groundwater age. Solute-contour lines represent groundwater travel time, which can then be coupled with Geographic Information System analyses to examine the relationship between water quality and historical land-use patterns. The reverse flow and solute modelling produced a reasonable distribution of groundwater travel times across the watershed, given the hydrology of the system. These groundwater flow paths would be unexpected if surface topography or even surface hydrology were used to predict groundwater movement. Approximately 70% of the watershed has a groundwater lag of ≤ 30 years. When the temporal lags for individual drainage areas within the watershed are compared, flush times vary dramatically. This variability is related both to the size of the sourceshed and its geology. The influence of a particular land use on stream chemistry changes depending on the time scale considered, and also depending on the sourceshed in question as a result of landscape diversity. The results suggest that land-use management practices to reduce solute loading to a watershed might not result in water-quality improvements for many years, especially if implemented on land far from streams. The influence of long groundwater flow paths that integrate past and current land uses must be considered in the interpretation of land-use effects on surface-water quality.