Publication Summary

A preliminary hydrogeologic study was conducted at the WDOT-owned site adjacent to Highway 101 on the Skokomish Indian Reservation in Mason County from June 1999 to May 2000, to evaluate the suitability of the site for rapid infiltration of treated municipal effluent. This report represents the first of two volumes that together assess the site suitability. Volume 2 (Pitz, 2000) describes the potential for groundwater mounding at the site based on the hydrogeologic characterization and proposed design conditions.

A preliminary hydrogeologic study was conducted at the WDOT-owned site adjacent to Highway 101 on the Skokomish Indian Reservation in Mason County from June 1999 to May 2000, to evaluate the suitability of the site for rapid infiltration of treated municipal effluent. This report represents the first of two volumes that together assess the site suitability. Volume 2 (Pitz, 2000) describes the potential for groundwater mounding at the site based on the hydrogeologic characterization and proposed design conditions.

Four groundwater monitoring wells were installed at the site and sampled for water level (monthly) and water quality constituents (quarterly). Five test pits were logged, and samples from the pits were analyzed for grain size. Five percolation tests were conducted to evaluate soil permeability. Water level elevations at the downgradient wetland also were measured during the winter rainy season.

The unsaturated zone was 15-28 feet thick at the site. The seasonal water table fluctuation was 1.5-3.6 feet. Based on specific capacity tests, hydraulic conductivity was estimated to be 350-400 feet/day in the coarse, outwash material found at the center of the site and 60 feet/day in the westernmost well screened in finer material. Samples, field tests, and visual observations of the native soils indicate rapid permeability and good suitability for rapid infiltration. However, landslide debris soils recently imported onto a large portion of the site surface had very low permeability and are not suited for rapid infiltration. A lower permeability zone was also observed in the boring of the westernmost well and may indicate a contact zone with upslope deposits that are finer than the coarse, native deposits covering most of the site.

Groundwater quality at the site was similar to background concentrations in the area for pH, specific conductance, total dissolved solids, nitrate+nitrite-N, total phosphorus, and chloride. Fecal coliform bacteria were not detected. Petroleum hydrocarbons were also not detected in a single sample from the monitoring well immediately downgradient of the WDOT maintenance shop on the site.

Link to EIM data for User Study ID SKOKGW