Camp Casey Reclaimed Water Infiltration Feasibility

Island County, Washington

HWA was contracted by Davido Consulting Group to support them in a feasibility study of wastewater system upgrades. Three existing drainfields are located at the facility; two located on the low-lying Parade Grounds, and a third on an upland area on the northern portion of the facility. The facility intended to replace existing septic tank systems with a membrane bioreactor (MBR) wastewater treatment system and infiltrate the treated wastewater through existing and expanded on-site drainfields.

Soils at the site are predominantly low permeability marine, marsh and glacial deposits consisting of silts and silty sands. Site concerns included shallow ground water (up to one foot below grade during the winter) at low-lying areas of the site and potential slope stability issues associated with an upland drainfield location. HWA evaluated the existing drainfield sites as well as alternative locations for additional capacity based on geologic, geographic, and functional criteria.

HWA conducted explorations at selected sites, including hand auger and hollow-stem auger borings, piezometer installation, and laboratory soil testing to determine grain size distribution. HWA estimated infiltration rates at selected sites based on the grain size testing results, as well as reviewing records of wastewater system discharge volumes to assess infiltration rates and existing capacity.

Based on these findings, HWA determined that the existing drainfields have the capacity to accept the additional design flow. Additionally, HWA assessed agronomic loading rates for use of reclaimed water for irrigation during peak periods as a feasible capacity augmentation.