title: Hydrostratigraphy, groundwater flow, and nitrate transport within the Abbotsford Sumas aquifer, Whatcom County, Washington
Creators: name: Stasney, David V.
Publisher: name: Western Washington University
Year: 2000

description: <p>From April 1997 to January 1999, a water quality study in a four square mile area in northern Whatcom County was conducted by the Geology Department and the Institute for Watershed Studies at Western Washington University. As part of this study, this thesis focused on characterizing the hydrostratigraphy, groundwater flow directions and flow velocities and developing a groundwater model using MODFLOW and Boss GMS version 2.0 (GMS) to simulate groundwater flow and nitrate transport within this area. Monthly groundwater levels and water quality data were collected and analyzed from 21 domestic wells and one piezometer in the field and at Western Washington University’s Institute for Watershed Studies State Certified Laboratory. This thesis used data collected from April 1997 to August 1998 which was divided into six seasons. To characterize the hydrostratigraphy, five geologic cross sections were constructed from well logs in the study area using Autocad. Six geologic units were identified, which were grouped into two hydrostratigraphic units defined as the Sumas Aquifer and the Sumas Aquitard. The Sumas Aquifer in this area is an unconfined aquifer ranging in thickness from 180 feet in the northeast portion of the study area to 80 feet in the southeast portion of the study area. The Sumas Aquifer is a heterogeneous mixture of gravel and sand with some scattered silt and clay lenses and displays good hydraulic continuity. The average hydraulic conductivity for the Sumas Aquifer was determined to be 929 feet/day. The Sumas Aquitard is primarily a clay layer underlying the Sumas Aquifer interpreted as Bellingham glaciomarine drift. The Sumas Aquitard also consists of scattered lenses of clay and or silt interpreted as ice contact and lacustrine deposits. Water table contour maps were created using the computer program Surfer version 6.0 (Surfer) for each of the six seasons using seasonally averaged water level data. A separate water table contour map was generated using the results of a groundwater model simulation. Groundwater flow directions determined from both sets of water table contours showed an overall northwest to southeast trend with the exception of the northwest portion of the study area which showed a south to southwest trend shifting to a southeast trend in the southeast portion of the study area. Groundwater flow velocity determined from seasonal water table contour maps using Surfer was approximately 20.0 feet/day in the northwest to southeast direction. The approximate travel time from the international border to the southern end of the study area in the direction of groundwater flow determined using field data was 1.8 years. Groundwater flow velocity using the model simulation was approximately 25 feet/day. The approximate travel time from the international border to the southern end of the study area in the direction of groundwater flow using GMS was approximately 1.5 years. Groundwater levels varied from season to season with the largest average difference of 4.4 feet (throughout the study area) occurring between Spring 1997 and Fall 1997. The greatest variation in water level in any one well due to seasonal recharge was 7.8 feet between Spring 1997 and Fall 1998 at well 3. Well 1 was the only well having a correlation between rising water levels and elevated nitrates. Two dimensional nitrate contours were created for each of the six seasons using seasonally averaged monthly nitrate data. Two dimensional nitrate contours indicate that the highest concentrations were dovm-gradient from large dairies and fertilized crops within the study area. Water quality data and two-dimensional nitrate contours indicate that most of the contamination in the central portion of the study area is localized and likely coming from sources up-gradient of wells 9, 14, and 18. However, elevated nitrates in wells 5, 6, and 7 in the northeast portion of the study area are attributed to sources across the international border in Canada. A groundwater model was developed for the study area using MODFLOW and GMS. Three nitrate transport simulations were created using GMS and a transport model (MT3D). A 50 mg/L spike of nitrate was entered into the model domain at selected points for one day. The nitrate spikes created contaminant plumes which were contoured at the end of one year for scenarios one and three, and at the end of six months for scenario two. Comparing nitrate transport simulations with nitrate concentrations obtained in the field revealed correlations of elevated nitrates from known up-gradient loading sources. Nitrate transport simulations indicate that large nitrate concentrations in the north and south central portions of the study area are likely caused by local source loading rather than source loading in Canada (specifically wells 14, and 18). However, transport simulations also suggest that the relatively stable elevated nitrate concentrations in the northwest (specifically wells 1 and 2) and northeast (specifically wells 5 and 6) portions of the study area are likely caused from sources across the international border in Canada.</p>