Introduction
Groundwater discharge to surface water can transfer contamination from groundwater to streams. This paper describes conditions where the ratio of groundwater contributing to surface water flows increased with increased rainfall and catchment wetting. This is not consistent with the generally accepted model of groundwater discharge dominating streamflow in dry periods; rather, it highlights the dynamics of the groundwater discharge process.
Methodology
Groundwater discharge processes were assessed at sites where groundwater and surface water chemistry allowed their relative inputs to be evaluated. Initial “spot” monitoring events indicated that, at times, surface water quality became more similar to groundwater even when surface water flows had increased. Sites that were instrumented with level, and in some locations EC, loggers in groundwater and streams provided increased understanding of groundwater discharge dynamics.
Results
At one site, spring discharge became more consistent with groundwater than surface water in periods of higher rainfall, indicating that hydraulic loading of the catchment increased groundwater discharge to the surface. At an intermittent flowing stream instrumented with EC and level loggers, after stream flows had been established for several weeks, later increases in surface water salinity continued to occur. This was considered to be related to further wetting of the catchment increasing hydraulic gradients and, consequently, groundwater discharge, even with higher stream flow rates. At a third site, during a temperate winter, surface water salinity continued to increase in response to increasing groundwater discharge until sufficient rainfall events had occurred to reverse hydraulic gradients between the stream and groundwater.
Summary and conclusions
The results highlight the importance of temporal monitoring of stream and groundwater systems to assess the dynamics of groundwater discharge to surface water. Level and EC logger data provide a framework in which to assess temporal variability of groundwater discharge under different surface flow conditions.