Publication Summary

A complex suite of biogeochemical processes can occur below the sediment surface in aquatic environments. These processes can produce strong vertical concentration gradients in upwelling groundwater, and significantly alter the chemical character of groundwater discharge to surface water. This paper presents a refined field method for collecting high-resolution porewater samples from immediately below the groundwater/surface water interface to help characterize these changes. The procedures can help to improve the accuracy of estimates of chemical loading to surface water by groundwater discharge.

A complex suite of biogeochemical processes can occur below the sediment surface in aquatic environments. These processes can produce strong vertical concentration gradients in upwelling groundwater, and significantly alter the chemical character of groundwater discharging to surface water. Accurate field measurements of these changes can be important for studies that depend on estimates of groundwater discharge chemistry and pollutant loading.

This paper presents a refined field method for high-resolution water quality sampling of porewater in shallow sediments underlying the groundwater/surface water interface. A programmable syringe pump was coupled with an M.H.E. Inc. PushPoint device to collect porewater samples using an ultra-low-flow (=2.5 ml/min) approach.

During October 2008, the method was field tested in Lake Whatcom at a location previously sampled using traditional in-water piezometers. This ultra-low-flow method was successful in collecting unbiased, depth-discrete porewater samples at a 5-cm resolution, and revealed a significant reduction in dissolved phosphorus concentration in the uppermost 50 cm of the study area sediments.

The field method described provides a low-cost, easy-to-use alternative to previous methods developed for porewater profile sampling. The method can help to reduce uncertainty and improve the overall accuracy of the Total Maximum Daily Load loading assessments and numerical modeling efforts conducted by the Department of Ecology′s Environmental Assessment Program. This technique may also benefit a variety of other projects where groundwater chemical loading to surface water is of concern.

Link to EIM data for User Study ID CHPI003