Port of Sunnyside
Project: Wastewater Irrigation Water Quality Assessment
Location: Sunnyside, Washington
Excessive concentrations of salts—chloride, dissolved solids, and
nitrate—were detected in the groundwater beneath 400 acres of alfalfa
that had been irrigated with food-processing wastewater. This degradation
in water quality had not only affected crop yields, but it had also prompted
a state order to mitigate the problem.
Salt concentrations in groundwater samples taken from the site varied
widely over time and space, making interpretations difficult. In addition,
drainage patterns had been altered by human activity, which included constructing
water quality treatment ponds on the site. Furthermore, identifying the
Port’s contribution to the problem was difficult because agricultural
activity was intense in surrounding areas.
PGG was part of a team that evaluated these water quality issues and assessed
mitigation strategies. Our role in the project entailed analyzing hydraulic
conductivity, water level, and water quality data from nearly 100 wells
and piezometers. This data was essential to understanding groundwater
flow directions and rates as well as contaminant fate and transport mechanisms.
We also developed a detailed, daily soil-water balance and a water quality
database. By identifying and monitoring an area that was isolated from
off-site influences, we were able to determine how the irrigation practices
contributed to the water quality problems.
We discovered that nitrate problems were probably related to the oxidation
of historical wetlands—a process outside the Port’s control.
We also concluded that irrigating with the highly saline wastewaters could
not continue without further violating groundwater quality standards for
chloride. Our recommendations motivated the Port to work with its industrial
customers to reduce chloride discharges. Finally, our study showed that
the Port could increase alfalfa yields by modifying its irrigation practices.