Lake Spokane

Groundwater non-point source input of phosphorus to Lake Spokane, Washington - Phase 2

Problem: In recent decades, Lake Spokane has experienced water-quality problems associated with eutrophication, and phosphorus was identified as the limiting nutrient that regulates the growth of aquatic plants in the lake. Phosphorus is delivered to Lake Spokane from municipal and industrial point-source inputs to the Spokane River upstream of Lake Spokane, but is also conveyed by groundwater and surface water from nonpoint-sources including septic tanks, agricultural fields, and wildlife. In response, the Washington State Department of Ecology listed Lake Spokane on the 303(d) list of impaired water bodies for low dissolved oxygen levels and developed a Total Maximum Daily Load (TMDL) for phosphorus in 1992, which was revised in 2010 after continued algal blooms and water-quality concerns. Between 1992 and 2010, point sources of phosphorus were reduced and now groundwater inputs from on-site septic systems have been identified as a potential substantial source of phosphorus to Lake Spokane. Current estimates of phosphorus loading to Lake Spokane from discharging groundwater are not well defined, but are needed to facilitate management and reduction of sources of phosphorus to the lake and its biota.

Objectives: The objective of this study is to estimate the groundwater input of phosphorus, including seasonal variations, to Lake Spokane by measuring the concentration of phosphorus in groundwater and estimating groundwater discharge rates. Quarterly groundwater-quality measurements and groundwater-discharge rate estimates will be used to estimate seasonal variations in groundwater inputs of phosphorus.

Relevance and Benefits: This study directly supports the U.S. Geological Survey (USGS) mission and goals related to water-resources issues identified in the USGS Science Strategy to monitor and report on the state of the Nation's terrestrial and freshwater ecosystems and to study the causes and consequences of ecological change. The findings of the study will provide resource managers with the information to support a decision on whether installation of a new sewer collection and treatment system is warranted in order to protect human health, and improve habitat for biota in Lake Spokane.

Approach: This project will focus on sampling near-shore groundwater for 2 years (quarterly) at 20 to 30 locations throughout Lake Spokane, and analyzing the samples for phosphorus. Sample locations will be located downslope from low to high density residential areas. Measured phosphorus concentrations will be combined with estimates of groundwater flux into the lake to quantify shallow groundwater phosphorus loads into the nearshore. Water chemistry will be analyzed from these locations in filtered groundwater collected from shallow piezometers that are hand-driven to depths of approximately one meter. Groundwater flux estimates will use established techniques including seepage meters and heat-flux modeling of vertical temperature profiles obtained by data-logging thermistors deployed in shallow piezometers.

YG00FS8 - Groundwater non-point source input of phosphorus to Lake Spokane, Washington - Completed FY2016

Problem: Lake Spokane, a 24 mile reach of the Spokane River between Nine Mile and Long Lake Dams, has been experiencing water quality problems for decades, along with other segments of the Spokane River. Dissolved oxygen in Lake Spokane is seasonally impaired, in part due to decay of the summertime bloom of plant material that results from the input of nutrients from domestic and municipal sewage. The limiting nutrient that most stimulates the growth of aquatic plants in Lake Spokane is phosphorus. Significant progress has been made at reducing point-source inputs of phosphorus to the Spokane River from municipal and industrial sources upstream of Lake Spokane. Nonpoint sources, including drainage from on-site septic systems, have thus become more significant components of total nutrient inputs. Phosphorus from septic system effluent is partially retained by soils beneath septic system drain-field via sorption and precipitation to sediment particles and may be further attenuated by sorption during groundwater flow. However, there is a growing concern that the retention capacity of sediment particles for binding phosphorus can be exceeded allowing phosphorus in septic system effluent to migrate greater distances potentially discharging to surface water. As such, groundwater inputs from on-site septic systems have been identified as a potential substantial source of phosphorus to Lake Spokane based on the very coarse textured sediments in the area, high density of septic system use in some areas, and a theoretical analysis of phosphorus retentions capacity for this area. Previous attempts to identify septic influences to Lake Spokane by analysis of lake water for optical brighteners in laundry detergents have been inconclusive. Current data are needed on the concentration of phosphorus in septic-influenced groundwater that is discharging to Lake Spokane to facilitate management and reduction of those sources.

Objectives: The overall objective of this investigation is to better assess the groundwater input of phosphorus to Lake Spokane from onsite septic-systems. In the first phase of this investigation covered by this proposal, the objective is to identify groundwater inputs along the north shore of Lake Spokane that are influenced by septic system effluent. Additionally, groundwater concentrations of phosphorus will be measured along a range of land use and septic-system densities. Subsequent investigations will address temporal variations of groundwater and nutrient fluxes to Lake Spokane.

Relevance and Benefits: This study directly supports the USGS mission and goals related to water-resources issues identified in the USGS Science Strategy to monitor and report on the state of the Nation's terrestrial and freshwater ecosystems and to study the causes and consequences of ecological change. The findings of the study proposed will provide resource managers with the information to support a decision on whether installation of a new sewer collection and treatment system as an effective means by which to protect human health, and improve habitat for biota in Lake Spokane. The methods proposed to be used to identify septic-influenced groundwater in this study are expected to be applicable to lakes throughout the region.

Approach: Phosphorus concentration in discharging groundwater will be measured from areas with a range of suspected levels of septic-influence. Phase I of this study will focus on identifying areas of septic-influenced groundwater discharge by mapping the nitrogen-isotope ratios of rooted aquatic plant tissue growing in the near-shore littoral regions of Lake Spokane. Groundwater nitrate originating from septic systems is enriched with ¹⁵N due to biological fractionation relative to nitrate originating from other sources such as fertilizers. The aquatic plants incorporate nitrogen over time, so areas where septic system effluent discharges are expected to have enriched ¹⁵N of plant tissue. Once the areas of high septic influence have been identified, samples of shallow groundwater discharging to the lake will be collected and analyzed for nutrients and selected other septic constituents. The results of the phosphorus and other nutrient analyses will be statistically analyzed based on land-use classification and density of the upgradient septic systems.