Groundwater Under
the Pacific Northwest • November 2-3, 2005 • Stevenson, Washington
ABSTRACT
Presenter:
Barbara J. Rich, Deschutes
County Environmental
Health Division
Co-Authors: Stephen R. Hinkle, David S.
Morgan
The La Pine
sub-basin of the Upper
Deschutes River
is underlain by a shallow sole source aquifer that currently supplies drinking
water for approximately 18,000 people.
Fifteen thousand lots of one-half to one acre in size were platted in
the 1960s and 1970s, prior to Oregon’s land
use planning laws, located within a 125-square mile corridor near the scenic Deschutes River and the smaller Little Deschutes
River. Without an understanding of the
high water table or the aquifer’s water quality and with no promise of public
infrastructure, the lots were marketed nationally at the time to unsuspecting
buyers. At build-out, this area could
support as many as 26,000 people.
The groundwater
quality in this region of southern Deschutes
County, Oregon is
threatened by nitrate contamination from onsite wastewater as a result of rapid
rural residential growth over the past 30 years. Based on data collected over the past 10
years, we now know that the soils in this region are highly porous and
permeable with no intervening layer protecting the shallow unconfined
aquifer. These young pumice soils are
low in organic matter and rapidly draining, which results in the rapid downward
movement of recharge from precipitation.
However, the aquifer receives only about 2.0 inches of total annual
precipitation, and so resulting groundwater velocities are low. The shallow portion of the aquifer, to a
depth of about 50 feet below the water table, is highly oxygenated. Below this, the aquifer becomes suboxic and, coupled with buried carbon sources, becomes a
natural denitrifying system. This
natural denitrification protects portions of the La
Pine aquifer from nitrate accumulation, but the upper oxic
portions remain vulnerable to anthropogenic nitrogen.
Extensive work by
the U.S. Geological Survey (USGS), in cooperation with the Oregon Department of
Environmental Quality (DEQ) and Deschutes County Environmental Health Division
(DCEH), resulted in a detailed investigation of the
geochemical processes at work and the development of a three-dimensional
groundwater and nutrient fate and transport model for the sub-basin. The USGS subsequently enhanced the
functionality of the 3-D model by linking it with an optimization model
developed to define specific management scenarios for the region based on
environmental or other constraints.
These constraints may include maximum groundwater nitrate levels,
maximum discharges allowed to the river systems and the treatment levels
attainable by onsite wastewater treatment systems.
This presentation
illustrates several aspects of the policy development process, including the
development of science based public policy and regulation in a
multi-jurisdictional setting; the use of the NLMM and other results of the
scientific investigations to educate local businesses, property owners, and
other stakeholders; and the potential for tracking the results of the
management activities in terms of environmental outcomes.