Department of Geology
Eastern Washington University

Recent Masters theses in Geology

HYDROGEOLOGY OF URBAN WATERSHEDS IN CHENEY, WASHINGTON, WITH AN EMPHASIS ON
STORMWATER RUNOFF AND WATER QUALITY

Douglas H. Christenson

Abstract

	An investigation of surface runoff water quality and flow rates was
conducted in preparation of a stormwater management plan for Cheney,
Washington.  Four main factors were analyzed for this study:  (1) inflow,
as actual precipitation and as two-year and 25-year storm intensities;/ (2)
water quality of surface runoff; (3) outflow measured at runoff discharge
sites; and (4) hydrologic analyses of over 100 subbasins within Cheney,
including relative runoff potentials and calculated peak discharge rates.
	Precipitation data for study year 1995 includes daily rainfall
measured in Cheney.  The 1995 total of 23.5 inches of precipitation in
Cheney is approximately 40% above the average of 16.7 inches.  Several
months had short-duration precipitation intensities that approached the
two-year peak intensity rate for this area, but rainfall was otherwise
widely distributed throughout the year, with five months having
precipitation 0.5 inch or more above the norm.
	Soils with inherently low resistance to erosion on the hills at the
northwest edge of Cheney contribute sediments into runoff water,
accumulating in ditches and catch basins.  Shallow soils with low
permeability restrict percolation near discharge sites on the southeast
side of town, where uncontrolled ponding of City runoff is an issue.
	Six drainage basin discharge sites were monitored for water quality
and outflow rates.  Surface runoff pH values were general slightly
alkaline, and hardness vales averaged 137 mg/L, which would classify the
runoff as hard water.  Site 2 at the northeast end of Cheney was the
exception, with consistently very soft runoff water.  The average zinc
concentration at Site 2 was above the criteria threshold for aquatic life
acute exposure (WAC, 1992).  Phosphorus concentrations were elevated at all
six sampling sites, especially Sites 1 and 2, which drain directly into
Minnie Creek.  Nickel concentrations at the six sites during the March 21
sampling session averaged 0.48 mg/L, which is well above the criteria
threshold concentration for aquatic life chronic exposure (WAC, 1992).
	Surface runoff flows measured at the basin discharge sites during
rainy periods show Basin 3 (the largest basin; located at center of town)
had the highest flows, averaging 0.86 c.f.s. with the highest being 2.97
c.f.s.  Basin 6 (the third-smallest basin, located at west end of town) had
the lowest flows, averaging 0.13 c.f.s.
	A rational formula hydrology spreadsheet was developed to calculate
two-year and 25-year peak discharges for each of 111 subbasins.  Individual
subbasin peak discharges ranged from 0.62 c.f.s. to 20.68 c.f.s. for a
two-year storm event, and from 1.63 c.f.s. to 20.68 c.f.s. for a 25-year
event.  The average calculated peak discharge rates were highest in Basin
3, averaging 6.66 c.f.s. for the two-year peak flow and 17.86 c.f.s. for
the 25-year peak flow.  Basin 3 also had the highest composite runoff
coefficient, 0.47, indicating a higher proportion of rainfall will end up
as surface runoff.
	The City's primary stormwater concerns are (1) contaminants in
runoff potentially impacting Minnie Creek and wetlands areas, (2)
uncontrolled outflows at and below the surface runoff discharge sites, and
(3) sedimentation throughout the stormwater conveyance system.  Surface
runoff to at least one discharge (Basin 3, with the highest measured
outflow) could be directed to a constructed wetland for water quality
enhancement and storage needs.