Scientific Investigations Report 2007–5237
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5237
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Rural residential areas near La Pine in southern Deschutes County and northern Klamath County, Oregon (fig. 1) have experienced rapid growth in recent years. More than 9,000 residential lots, ranging in size from 0.5 to 10 acres, are in the area where existing and future homes will rely on individual on-site wastewater systems and wells for wastewater disposal and water supply. Most existing wells have screened intervals within 50 ft of land surface and extract water from alluvial sands and gravels that constitute the primary aquifer in the area. The water table is shallow, typically is less than 20 ft below land surface, and in some low-lying areas rises seasonally to within 2 ft of land surface. Sandy soils derived from pumice contain little organic matter and allow rapid infiltration of on-site wastewater effluent.
The vulnerability of the shallow aquifer has led to concern by County and State land-use and environmental-health regulators that ground-water quality may be impaired if development continues at planned densities using conventional on-site wastewater systems. (In this report, conventional on-site systems include standard, pressure-distribution, and packed-bed [sand] filter systems). Another potential concern is the quality of local streams. The Deschutes and Little Deschutes Rivers, which flow through developed areas near La Pine, have been listed as “water-quality impaired” for temperature and turbidity; nutrient loading from ground water has been identified as a potential contributor to excessive algal growth in some reaches (Anderson, 2000; Jones, 2003) that may exacerbate water-quality concerns.
On-site wastewater systems are the principal source of nitrogen to the shallow ground-water system in the La Pine area (Century West Engineering, 1982; Oregon Department of Environmental Quality, 1994; Hinkle and others, 2007a). On-site wastewater systems do not remove nitrogen from wastewater; however, most nitrogen is converted from organic nitrogen and ammonium to nitrate before it reaches the saturated part of the ground-water system (water table). Once in the saturated zone, nitrate generally is stable in the presence of oxic ground water (ground water that contains dissolved oxygen). Adsorption or chemical reactions within the flow system do not readily remove nitrate. Nitrate is a human health concern because it can cause methemoglobinemia (Blue-Baby Syndrome) in infants (http://www.atsdr.cdc.gov/HEC/CSEM/nitrate/). Nitrogen also is an environmental concern as a potential source of nutrient enrichment to streams. Nutrient enrichment contributes to algal blooms detected in the Deschutes and Little Deschutes Rivers. The U.S. Environmental Protection Agency (USEPA) has established 10 mg N/L as the maximum allowable nitrate concentration in drinking water for public water supply systems. Oregon, by statute, has established a nitrate concentration of 7 mg N/L as the value at which action may be taken to control water-quality degradation by regulatory means.
The present location of the city of La Pine (fig. 1) was the first concentrated development within the study area. The first building permits recorded in what was then called the “core area” date from 1910. In 2006, the core area was incorporated as the city of La Pine. Degradation of ground-water quality by on-site wastewater systems was first documented in the core area in 1979 by Oregon Department of Environmental Quality (ODEQ) after samples from 46 wells revealed that water from 8 wells contained nitrate concentrations greater than 10 mg N/L (Cole, 2006) and a maximum of 26 mg N/L. Nitrate concentrations in ground water as much as 41 mg N/L were detected in a follow-up study in 1982 (Century West Engineering, 1982). These results led to an ODEQ administrative rule requiring community sewage collection, treatment, and disposal for the core area. In 1993, ODEQ sampled 36 wells in residential areas near La Pine as part of its statewide Ambient Groundwater Monitoring Program and detected nitrate concentrations greater than 2 mg N/L in 19 wells. They concluded that elevated concentrations were caused by anthropogenic influences (on-site wastewater systems) (Cole, 2006). Concentrations were greater than 10 mg N/L in 4 (11 percent) of the 36 wells sampled. In 1994, Deschutes County requested that ODEQ further evaluate ground-water quality in the area. Water from more than 120 domestic and public water-supply wells was sampled in 1994 and 1995 and ODEQ delineated several areas of elevated nitrate concentrations underlying the most densely developed parts of the region (R.J. Weick, ODEQ, written commun., 1998; Cole, 2006). As part of the 1995 assessment, ODEQ constructed simplified nitrate transport models which included basic assumptions on nitrogen chemistry. The ODEQ transport models predicted that nitrate concentrations would exceed drinking water standards within 20 years of full build-out (R.J. Weick, ODEQ, written commun., 1998; Cole, 2006).
In 1999, a nitrate concentration threshold of 5 mg N/L was selected for the La Pine area by the Deschutes County Working Group on Groundwater Issues for the South Deschutes Basin to serve as a “proactive target to protect the Basin’s groundwater quality.” The working group also recommended that ODEQ “…address these problems and concerns by considering the adoption of a geographic rule…to protect the Basin’s groundwater resource.” (http://www.co.deschutes.or.us/download.cfm?DownloadFile=507DDA68-BDBD-57C1-902813CCF281DDF0).
In 1999, Deschutes County and ODEQ identified the need for an improved understanding of the processes that affect the transport and fate of nitrogen in the La Pine area before making decisions among alternatives for managing ground-water quality. To help achieve that understanding, Deschutes County and ODEQ applied for and received funding from the USEPA to evaluate methods to protect ground-water quality in the area (Oregon Department of Environmental Quality, 2004a) as part of the La Pine National On-Site Wastewater Demonstration Project (NDP). The objectives of the NDP also included (1) assessing the effectiveness of advanced treatment (denitrifying) systems for on-site wastewater and (2) developing a more complete and useful understanding of processes that affect nitrogen in the ground-water system. The second objective was the subject of a cooperative study by the ODEQ, Deschutes County Community Development Department (CDD) and the Oregon Water Science Center of the U.S. Geological Survey (USGS).
The primary objective of this study was to develop a thorough understanding of the hydrologic and chemical processes that affect the movement and fate of nitrogen within the shallow aquifers of the La Pine region. A secondary objective was to provide a method for analyzing the effects of existing and future development on water quality. This understanding will provide local and State resource management agencies with information and tools needed to determine the probable effects of present and future land use on nitrogen concentrations in the shallow aquifer and on nitrogen loading from ground water to the Deschutes and Little Deschutes Rivers.
This report describes conceptual, simulation, and management models of the ground-water system near the community of La Pine in central Oregon. The description of the conceptual model provides a background for the development of the simulation and management models and includes the geologic framework, hydrologic processes, and processes affecting nitrate transport and fate. The description of the simulation model includes a discussion of how the processes and boundaries of the system were represented in a computer simulation model, how the model was calibrated, and the results of predictive simulations. The simulation model was enhanced as a decision-support tool (management model) by incorporating optimization techniques that allow users to identify management solutions that will meet water quality goals. The discussion of the management model includes a description of the method of incorporating optimization techniques with the simulation model, the formulation of the management problem, and analysis of the sensitivity of optimal management solutions to the values of various constraints.
The La Pine study area encompasses about 250 mi2 within the Deschutes River drainage basin in central Oregon (fig. 1). The area is drained by the Deschutes River and tributaries including the Little Deschutes, Spring, and Fall Rivers. Land-surface elevation ranges from about 4,000 ft near the Deschutes River at the northern boundary of the study area, to nearly 5,700 ft at the peaks of volcanic buttes in the northwestern corner of the study area. Most of the study area lies in the relatively low-relief alluvial plain of the Deschutes and Little Deschutes Rivers at elevations between 4,150 and 4,300 ft. The populated areas in the study area include low and medium density rural-residential subdivisions adjacent to and between the Deschutes and Little Deschutes Rivers extending northward from La Pine to the community of Sunriver (fig. 1). The subdivisions are surrounded by Federal lands managed by the U.S. Forest Service and Bureau of Land Management.
The study area boundaries were selected to include the most densely populated parts of southern Deschutes County and northern Klamath County, where on-site wastewater systems are the predominant method of wastewater disposal for existing and future residential development. The boundaries also were selected to include the principal area of the shallow alluvial aquifer that provides drinking water for most of the population in the La Pine area. The data collection and analysis for this study was focused in the 247 mi2 area of the study-area flow and transport simulation model (fig. 1).
Residential development in the area began to accelerate in the 1960s (Century West Engineering, 1982) and almost 3,000 new residential lots were created in the 1970s in response to demand for vacation homes and full-time residences. Although lots range in size from 0.5 to more than 10 acres, 58 percent of lots in the study area in 2000 were less than 1 acre and 82 percent were less than 2 acres. The population of the area was less than 1,000 in 1960, and increased to about 5,600 in 1981 (Century West Engineering, 1982), and was approximately 14,000 in 2000 (U.S. Bureau of the Census, 2000).
Land use is primarily low- to medium-density residential. Commercial and medium density residential areas are in the incorporated City of La Pine (including the La Pine “core area” and Wickiup Junction area) and industrial development is immediately east of the core area. Agricultural lands cover less than 4 percent of the study area and most of that area is nonirrigated pasture (Tim Berg, Deschutes County Community Development Department, written commun., 2004).
The climate of the area primarily is controlled by eastward moving air masses from the Pacific Ocean. Orographic precipitation in the Cascade Range results in more than 200 in/yr in some locations, although precipitation rates in the lower part of the Deschutes River basin average as little as 10 in/yr. Mean annual (1971–2000) precipitation in the La Pine area ranges from 16 to 24 in. (Daly and Gibson, 2002).
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