Contaminant mass discharge to streams: Comparing direct groundwater velocity measurements and multi-level groundwater sampling with an in-stream approach

Quantitative knowledge of contaminant mass discharge is useful when assessing the risk posed to streams by point sources. However, due to a multi-directional flow field at the groundwater-surface water interface, reliable estimates close to streams are particularly challenging to obtain. Moreover, since the “true” value of the total contaminant mass discharge across a defined control plane is typically unknown at field sites, it is difficult to assess the accuracy of estimates at the field scale.

We estimated the mass discharge of a chlorinated ethene plume entering a low-land Danish stream across a control plane on the stream bank. This was done using multi-level groundwater sampling combined with 1) specific discharge obtained from Darcy’s law, and 2) two different specific discharge fields (a constant equal to the mean, and a varying) obtained from direct groundwater velocity measurements using point velocity probes (PVPs).

The methods yielded contaminant mass discharges ranging from 204 to 372 kg/y (PCE equivalents). To help account for the entire contaminant mass, we also quantified the contaminant mass discharge of chlorinated ethenes from measured contaminant concentrations in the stream water and the stream discharge to obtain an in-stream contaminant mass discharge.

By subtracting contaminant input upstream from the plume discharge zone, as well as input from two culverts discharging contaminated water to the stream, this estimate was used to calculate a groundwater-borne in-stream contaminant mass discharge of 558 kg/y (PCE equivalents). This method has not previously been explored to access the accuracy of stream bank contaminant mass discharge estimates.

Differences between the groundwater-borne in-stream value and those on the stream bank may be due to high concentration zones located in fast flow conduits or missed high concentration zones in the control plane. It is also possible that the groundwater-borne in-stream mass discharge was overestimated due to uncertainty in the input from two discharging culverts.

The agreement of these results is sufficient to be of practical usefulness for risk assessment. It is concluded that the combination of direct groundwater velocity measurements and multi-level groundwater sampling can provide a useful extension of Darcy-based methods when quantifying the contaminant mass discharge to streams.

Moreover, this study demonstrates the efficacy and usefulness of the in-stream contaminant mass discharge in risk assessment if fully mixed stream water concentration and corresponding stream discharge are available..