Evaluation of Nutrients in Durham County, North Carolina, Urban Streams

Science Center Objects

Falls Lake and Jordan Lake are included in the North Carolina Division of Water Quality's 303(d) list of "impaired" waters due to violations of the State chlorophyll-a standard caused by excessive nutrient inputs. Scientific data are needed allocate resources to achieve nutrient reduction targets have been adopted for the Falls Lake and Jordan Lake watersheds for stormwater, as well as for wastewater and agricultural inputs. 

This study evaluated the utility of stable isotopes for characterizing nutrient sources in selected tributaries in the vicinity of Durham that feed nutrient-impaired Falls Lake and Jordan Lake. Results provided a scientific basis for identifying sources of nutrients (commercial fertilizers, animal and septic waste, naturally occurring) and for optimizing the installation locations of nutrient-reduction Best Management Practices (BMPs) in the study area.

Nutrient Levels

Image of Third Fork Creek, Durham, North Carolina
Third Fork Creek, Durham, North Carolina

Falls Lake and Jordan Lake are included in the North Carolina Division of Water Quality's 303(d) listof "impaired" waters due to violations of the State chlorophyll-a standard caused by excessive nutrient inputs. The North Carolina Environmental Management Commission has adopted nutrient management strategies for both reservoirs that call for comprehensive controls to reduce nitrogen and phosphorus loads from significant sources in the watershed, including urban stormwater, wastewater, and agriculture.

The City of Durham, Public Works Department, Stormwater Services Division (DSS) is tasked with implementing best management practices (BMPs) to reduce nutrient inputs from stormwater in both the Falls Lake and Jordan Lake watersheds. BMPs are required for new development and existing development. Urban watersheds typically have diverse sources of nutrients and other pollutants; therefore, implementation of effective Best Management Practices (BMPs) is complex.

Cabin Branch Creek, Durham, North Carolina
Cabin Branch Creek, Durham, North Carolina

Stable isotope analysis

Stable isotopes are useful for determining the relative abundance of different nutrient sources in some settings. There are two stable isotopes of nitrogen: 14N and 15N. All nitrogen compounds contain both isotopes, but because of isotopic fractionation they are incorporated into compounds in differing ratios. Commercial fertilizers, animal and septic waste, precipitation, and organic nitrogen are common sources of nitrate, and each has a distinguishable isotopic 15N/14N ratio.

If multiple sources of nitrogen compounds are mixed together in surface runoff or in a water body, the ratio of light (14N) to heavy (15N) nitrogen isotopes in the water can be used to estimate the relative contributions of the sources.

Additionally, under some conditions, variations in isotopes of water (18O and 2H) affected by precipitation can be used to differentiate relatively "old" groundwater and the more variable "new" precipitation components of surface-water runoff. Thus, it may be possible to determine the contribution of groundwater discharge to a stream relative to overland runoff resulting from precipitation.

Approach:

The primary objective of this investigation is to evaluate nutrient sources in selected tributaries in the vicinity of Durham that feed nutrient-impaired Falls Lake and Jordan Lake.

Results will provide a scientific basis for identifying sources of nutrients (commercial fertilizers, animal and septic waste, naturally occurring) and for optimizing the installation locations of nutrient-reduction best management practices in the study area.

Detailed objectives of this investigation are to:

  1. Document and compare concentrations of nitrogen and phosphorus among 3 urban streams in the Falls Lake and Jordan Lake watersheds;
  2. Evaluate the utility of stable isotopes for characterizing nitrogen sources and transport pathways in urban, low-order streams of the Piedmont Physiographic Province.

Task 1. Determine in-stream nutrient concentrations

Water-quality monitoring stations will be established at three stream sites. Two of the sites are located on tributaries upstream from Falls Lake in the upper Neuse River basin. One site is upstream from Jordan Lake in the Cape Fear River basin. Sites are co-located with DSS sampling locations, which will add value to the data generated by both agencies.

Sampling frequency

  • Samples will be collected approximately monthly for the first year of the project, for a total of 12 samples per site.
  • The sampling strategy will strive to collect the majority of samples during baseflow conditions, with 2 to 4 samples collected during storm events. If a stream is not flowing during a scheduled visit, it will not be sampled.

Data collection and analysis

  • Instantaneous streamflow will be measured at each site during water-quality sampling.
  • No new continuous-record streamflow gages will be installed for this project; however, existing USGS streamflow and rainfall gages that are located within or near the study area will be used to track general hydrologic conditions during the study period.
  • Dissolved oxygen, water temperature, pH, and specific conductance will be measured in the field at the time of sampling.
  • Water samples will be analyzed by the USGS National Water Quality Laboratory for nitrogen fractions (ammonia, nitrite plus nitrate, and total ammonia plus organic nitrogen) and total phosphorus. (Table 2)

Task 2. Investigate nitrogen sources and transport pathways using stable isotopes

Information on nutrient sources, transport, and delivery to waterways is necessary to develop sound nutrient-reduction strategies.

  • Analysis of stable isotopes of nitrate (15N, 14N, and 18O) is a useful tool for differentiating nitrogen sources (for example, inorganic fertilizer vs. waste products) and transport and delivery mechanisms within watersheds.
  • Analysis of water-molecule isotopes (2H and 18O) yields information on water sources (for example, older deep groundwater vs. recent atmospheric deposition)

Nitrate and water isotope samples will be collected concurrently with the nutrient samples, approximately monthly at the three stream sites from June 2011 through May 2012, for a total of 12 samples. Isotope analyses will be performed at the USGS Menlo Park Isotope Tracers Laboratory. (Table 2)

Table of constituents to be analyzed in water samples
Table of constituents to be analyzed in water samples

Task 3. Prepare final report

The USGS will publish a final project report summarizing the data collected during the study and evaluating sources of nutrients within small urban catchments.

This report will include:

  • Comparisons of nitrogen- and phosphorus- fraction concentrations and stable isotope results for the 3 streams
  • Land use within the catchment area for each sampling site, compared to the relative source contributions
  • Relative source contribution of fertilizers, naturally occurring soil nitrate, and animal (including manure or septic) waste based on analysis of stable isotopes of nitrogen and oxygen samples
  • Water sources (such as groundwater, atmospheric water vapor, and meteoric water) based on analysis of the isotopic composition of water samples