Publication Summary

In support of a TMDL analysis for Lake Whatcom, a study was conducted to examine the role of groundwater inflow in the lake′s nutrient budget. Groundwater samples were collected just prior to discharge to the lake using a network of lake-bed piezometers. Samples were tested for phosphorus and nitrogen nutrients, and additional indicator parameters. A median orthophosphate concentration of 0.06 mg/L was reported during the study period. An anthropogenic influence on the water quality condition is suggested for several of the stations tested. The results of this study are incorporated into a comprehensive analysis of lake loading published separately.

Lake Whatcom is listed on the 1998 Washington State 303(d) list of waterbodies not attaining the water quality criterion for dissolved oxygen. Evidence indicates that oxygen depletion in the lake is the result of several factors, including eutrophication processes driven by the availability of phosphorus and nitrogen-based nutrients. A Total Maximum Daily Load study has been initiated in order to develop a water quality model and make recommendations for reduction and allocation of pollutant loads for the lake.

To characterize the role of groundwater-derived nutrient inflow in the lake′s nutrient budget, a network of 14 lake-bed piezometers was installed in the near-shore sediments around the lake perimeter. Samples of groundwater seepage to the lake were collected from the piezometers quarterly from October 2002 through August 2003. Samples were analyzed for target nutrients -- total dissolved phosphorus, orthophosphate-P, nitrate+nitrite-N, and ammonia-N -- as well as dissolved oxygen, iron, manganese, total dissolved solids, chloride, dissolved organic carbon, and boron.

The median dissolved orthophosphate-P concentration observed in discharging groundwater was 0.06 mg/L, with a maximum reported value of 0.35 mg/L. The median total dissolved phosphorus concentration was 0.14 mg/L, with a maximum value of 2.18 mg/L. Nitrogen was normally present in the form of ammonia, with a median concentration of 0.21 mg/L as N. At several stations, the groundwater quality profile and upgradient land-use development patterns suggest an anthropogenic (human-related) origin for the elevated nutrients observed. In other cases, the data suggest the nutrients are likely associated with the decomposition of naturally occurring organic matter present in the aquifer sediments. Loading calculations for the 2003 water year estimate an upper-bound orthophosphate-P mass flux to the lake ranging from 900 to 1300 kgop via direct groundwater inflow. Changes in redox conditions in the final portion of the groundwater flow path may attenuate a significant percentage of the estimated load.

Link to EIM data for User Study ID CHPI002