B-C Creek Hydrogeology

Spike Minczeski

The B-C Creek is the drainage creek for the B and C parking lots for The Evergreen State College in Olympia Washington. During the 1999-2000 school term, a temporary taskforce was assigned the job of determining whether additional parking was needed to help facilitate a rising enrollment rate. This taskforce was asked to investigate a number of solutions and any environmental impacts those solutions might have. It was assumed that the B-C creek would carry the runoff if another parking lot were constructed. Several hydrology experiments were started in an attempt to measure existing conditions: to measure the geology, hydrology of the water table, interception rates, stream rates and any interesting scenarios that occur normally.  One experiment dealt with the instillation of a series of piezometers with which the water table and its geology could be analyzed.

The piezometers were installed at 200ft distances from each other, with the first series 200ft from the head of the creek at B-lot. The nests were broken up into pairs, with one on each side of the creek at a mean distance of 15ft perpendicular from the creek.  This was done to insure that the piezometers would not receive water from the creek, which could potentially bias data. Each piezometer was made of 2’ PVC piping, with caps on both ends. Strainers were cut into one end of each pipe using a hacksaw to allow water to enter and exit the pipe. A 2.5’’ hand auger was used to bore the holes. Each hole took between .5 and 1.5 hours to excavate. Once dug, pea gravel was pored into the hole to provide a base to rest the pipe on. Once the pipe was inserted, more pea gravel was poured around the pipe to create a highly transmissivic layer that would keep smaller sediment from clogging the strainers.  Clay and other earth was tightly packed around the remaining circumference of the pipe to create water-resistant seal.

The geology of the area indicated that it had once been the floor of a meandering stream, due to the presence of alluvial material such as course sand and gravel. In addition, layers of clay were continually intermixed, creating a small confining layer for the water table in the area. At the piezometers further downstream, the holes drilled were much shallower due to a higher water table, yet the soil showed a similar consistency.

The topography of the area slopes from north to south, with a hill in the near north area. It is believed that water is transported along the topography to the naturally low area surrounding the creek. This phenomenon of water following the topography is believed to occur between 3-8ft below the surface at a relatively accelerated rate, due to the presence of transmisive soils on a gradient, over a confining layer that prevents further saturation.

During the continued testing of the piezometers, several hypotheses were created to explain the hydrogeology. It was interpolated that the creek and the shallow aquifer were existing in equilibrium with each other, through continually gaining and loosing water from each other. During dry times, the creek level only drop a few inches, while the aquifer would show lower water levels, indicating that the stream was gaining from the aquifer. In wet times, the aquifer would quickly rise, with a steadily rising creek elevation, suggesting that the creek was loosing to the aquifer. This was much more prevalent on the south side of the creek, yet not quite as evident further downstream.

The water table in the areas downstream is high. During some storm events, the water elevation was only a few inches below the surface. Further downstream, some areas of groundwater flooding were evident. This situation is due to large flow in a topographically low region, which is an ideal situation for ground storage. If a new parking lot were built, this region would be at highest risk of environmental alteration because of an increased flow.