No abstract

The relationship between land use and stream chemistry is often explored through synoptic sampling of rivers at baseflow conditions. However, baseflow chemistry is likely to vary temporally and spatially with land use. The purpose of our study is to examine the usefulness of the synoptic sampling approach for identifying the relationship between complex land use configurations and stream water quality. This study compares biogeochemical data from three synoptic sampling events representing the temporal variability of baseflow chemistry and land use using R-mode factor analysis. Separate R-mode factor analyses of the data from individual sampling events yielded only two consistent factors. Agricultural activity was associated with elevated levels of Ca2+, Mg2+, alkalinity, and frequently K+, SO4(2-), and NO3-. Urban areas were associated with higher concentrations of Na+, K+, and Cl-. Other retained factors were not consistent among sampling events, and some factors were difficult to interpret in the context of biogeochemical sources and processes. When all data were combined, further associations were revealed such as an inverse relationship between the proportion of wetlands and stream nitrate concentrations. We also found that barren lands were associated with elevated sulfate levels. This research suggests that an individual sampling event is unlikely to characterize adequately the complex processes controlling interactions between land use and stream chemistry. Combining data collected over two years during three synoptic sampling events appears to enhance our ability to understand processes linking stream chemistry and land use.

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.

Forested riparian areas are believed to be important for reducing nonpoint source pollutants. These areas along streams, lakes, and wetlands have been reported to trap sediment and nutrients and enhance denitrification. Past research on the effectiveness of riparian areas has been based on existing forests rather than restored areas. An experiment using the paired-watershed design was established in northeastern Connecticut during 1992 to determine the water quality effects of reforestation on a riparian zone currently cropped in maize. Water quality fluxes in precipitation, overland flow, soil solution, groundwater, and streamflow were determined. Results indicate that this 35 m wide riparian zone had little attenuating influence on N concentrations in groundwater based on NO3−N concentrations and NO3−N:C1 ratios. The primary N flux to the stream was in the groundwater. Denitrification did not appear to be a major process operating in this system. Reforestation of this riparian buffer should result in improved surface and groundwater quality.