4. Fase II: Toma de datos
4.10 Redes Wireless
OVERVIEW
Studies of ground water quality have established a clear link between nitrate concentrations in ground water and land use. High nitrate concentrations are often associated with particular land use practices that can degrade water quality, such as septic system discharges and agricultural activities. Accordingly, nitrate is often used as an indicator of the risk of impairment to ground water quality related to land use and other activities, and is widely used as an indicator of overall water quality and the potential presence of biological (e.g., pathogenic) and chemical (e.g., pesticide, pharmaceuticals) contamination. High levels of nitrate in ground water can result in serious health impacts (e.g., methemoglobanemia, or “blue baby syndrome”) and can have long lasting effects on the geochemistry and ecological functioning of soils and water resources. Elevated concentrations of nitrate in surface water can cause a loss of biodiversity and lead to eutrophication, algal blooms and oxygen depletion. Primary nonpoint sources for nitrate are agricultural runoff and soil contamination from the introduction of septic system effluent and other diffuse loading sources. Point source discharges, such as those from sewage treatment plants also contribute to nitrate loadings.
Understanding and managing the impact of nitrate loadings is critical to maintaining the ecological water resource quality in the Highlands Region. In order to understand the existing quality of the ground water resources, it is necessary to determine the background nitrate concentrations using available water quality data. Elevated nitrate concentrations may be the result of development activities (e.g., installation of septic systems) and other sources of nitrate loadings like fertilizer applications, although naturally-occurring characteristics (e.g., limestone geology) can also contribute to elevated concentrations of nitrate because they allow for rapid transport of contaminants with little attenuation. The concentrations of nitrate in ground water in the Highlands Region vary from high water quality areas, where the measured nitrate concentrations are likely to be representative of natural conditions, to areas where intensive land use activities have resulted in elevated levels of nitrate concentration.
A critical component for protecting ground water quality is limiting discharges of wastewater to ground water. Potential water quality degradation can be mitigated with proper location, design, construction, installation, repair and operation of individual septic systems. If managed properly, these systems can provide for some ground water recharge. These systems are subject to the NJDEP Standards for Individual Subsurface Sewage Disposal Systems (N.J.A.C. 7:9A) and are generally regulated at the county and municipal level. An important issue in the proper placement of septic systems is the suitability of the soils on a site to handle septic system effluent, as septic systems are designed to utilize the soils as a functioning part of the treatment process. Although the soil conditions and geological characteristics of the Highlands Region provide some areas where the soil characteristics are appropriate for the use of septic systems, much of the Region’s soils are generally constrained for the use of conventional septic systems.
However, the Highlands Council recognizes that even with proper siting, design, operation and maintenance of septic systems, additional effluent discharges must be limited in order to prevent further degradation of water quality. This requires appropriate septic system density to ensure that future developed lots utilizing septic systems are appropriately zoned to provide for sufficient
Highlands Water Resources Technical Report Volume 1: Watersheds and Water Quality
115
dilution of effluent discharges. It should be emphasized that though nitrate serves as the indicator contaminant for septic systems, it also serves as a surrogate for other contaminants of concern that may also be present in septic system effluent, and protecting ground water quality against elevated nitrate concentrations also provides protection against other contaminants. To this end, the Highlands Council has developed a methodology for computing appropriate septic system densities for municipalities within the Highlands Region, and, based upon the land area available for development, computed the total number of allowable septic systems (i.e. septic system yields) per municipality and Land Use Capability Zone.
The Highlands Council’s methodology for computing appropriate septic system yields for different Land Use Capability (LUC) Zones at the municipal scale relies upon a number of different modeling approaches and analytical techniques that estimate at the subwatershed scale: 1) median nitrate concentration; 2) annual drought ground water recharge rate; 3) septic system density required for sufficient septic system effluent dilution, and 4) an estimate of developable land within each zone by municipality. Computing appropriate septic system densities first required estimating median nitrate concentrations in ground water at the subwatershed scale using statistical models. Based upon these estimated median concentrations, “target” nitrate concentrations for the Protection and Conservation Zones were established by the Council. The Trela-Douglas nitrate dilution model was then used to compute appropriate densities for each LUC Zone at the subwatershed scale, based upon target nitrate concentrations, assumed annual septic system nitrate loadings, and estimated annual drought recharge rates. Following computation of an appropriate septic system density, the number of additional allowable septic systems per Protection and Conservation Zones within the Planning Area was calculated for each municipality. In addition, the Highlands Council is also providing potential septic system yields for non-sewered portions of the Existing Community Zone. A necessary first step for computing appropriate septic system densities at the municipal scale is characterizing baseline conditions of nitrate concentrations throughout the Highlands Region. The Highlands Council analyzed nitrate concentrations in ground water measured in wells throughout the region, with consideration given to well location, construction, water use, land use, and available water quality data. An evaluation of specific well characteristics was performed to determine the data from wells that were representative of land use and water quality conditions in the Region. Based on analytical results from sampling of 352 wells selected for this analysis, the median of measured nitrate concentration for the entire Highlands Region is 1.1 milligrams/liter (mg/L). This analysis, while useful, provides a large scale characterization of ground water quality throughout the Highlands Region, and is biased towards more developed areas where wells with water quality data primarily exist. In order to better estimate the nitrate concentrations for undeveloped areas of the Region, for each of the 183 HUC14 subwatersheds within the Region, and for the Region as a whole (beyond the calculated median value), the U.S. Geological Survey (USGS) Water Science Unit with the support of the Highlands Council developed empirical-based logistic regression water quality models. This modeling approach examined anthropogenic and naturally-occurring variables potentially related to water quality such as urban, agricultural and other types of land use, soil characteristics, geologic features, hydrology and other factors, including septic system density, that could be used in statistical models to most reliably predict nitrate concentrations. The models used inputs primarily indicative of land use activities for estimating the probability that a pre-specified target nitrate concentration is exceeded. Model development incorporated an analysis of the available water quality data from 352 wells located in the Highlands Region, as well as land use variables and specific features that are predictive of changes in nitrate concentrations in ground water. The optimal set of predictive variables, as determined by modeling objectives and
Highlands Water Resources Technical Report Volume 1: Watersheds and Water Quality
116
performance, included: 1) the percentage of urban land use; 2) the percentage of agricultural land use; 3) the septic system density; 4) the length of streams; and 5) the number of known contaminated sites.
Based upon the median nitrate concentrations estimated with the logistic regression models for each of the 183 subwatersheds, the median concentration for the Highlands Region as a whole was determined to be 0.83 mg/L, slightly lower than the 1.1 mg/L value calculated directly from well sampling analytical results. The model-derived median is considered more accurate as it addresses limitations in the well monitoring network, related to the overall distribution of wells with a disproportionately small number located in undeveloped areas. The modeling analysis also provides an indication of general trends in water quality and magnitude of contamination in terms of both areal extent and actual concentrations that are related to nitrate loadings. Estimated median nitrate concentrations for each of the 183 subwatersheds range from 0.17 to 3.6 mg/L; just nine subwatersheds have an estimated median concentration greater than 2.0 mg/L. The median nitrate concentration in undeveloped areas was estimated to be 0.1 mg/L, with concentrations in subwatersheds with very limited development typically less than 1.0 mg/L. Highly urbanized areas are likely to have somewhat elevated concentrations, with intensely agricultural areas most likely to have the highest concentrations of nitrate. The results of the median nitrate concentration analysis, aggregated into representative values for the HUC14-specific results are illustrated in the map figure entitled Median Nitrate Concentrations by HUC14.
Highlands
Council
New Jersey 78 78 78 280 78 287 78 80 7878 78 287 7880$
Kilometers 0 5 10 15 Miles 0 10 20 Interstate Highways County Boundaries Highlands Region 0.17 to 0.72 0.73 to 1.87 1.88 to 3.23Median Nitrate Concentrations (Milligrams Per Liter)
MEDIAN NITRATE CONCENTRATIONS BY HUC 14
Scale = 1:555,000 Scale = 1:9,000,000 PA NJ NY CT MD DE
Highlands Water Resources Technical Report Volume 1: Watersheds and Water Quality
118
The median nitrate concentrations for the Protection and Conservation Zones within the planning areas of the Highlands Region were estimated to be 0.72 and 1.87 mg/L, respectively, and these concentrations were selected as the nitrate dilution targets for these two zones. By comparison, the median nitrate concentration estimated for the Existing Community Zone is 1.17 mg/L. For this zone, a nitrate target concentration of 2.0 mg/L, corresponding to NJDEP statewide target, was selected for the regional build out analysis regarding the limited parts of this Zone not served by public wastewater treatment systems. The selected target nitrate concentration of 2.0 mg/L reflects the protection and smart growth standards of the Existing Community Zone.
In addition to target nitrate concentrations, another model input variable required for estimating appropriate septic system densities is the annual ground water recharge rate. Annual ground water recharge under drought conditions was estimated for each subwatershed using the GSR-32 recharge methodology, which was developed by the NJ Geological Survey (Charles and others, 1993). The methodology is a soil-water budget approach that accounts for local climatic, soil, and land use/land cover characteristics to estimate annual ground water recharge using monthly time steps. In order to be most protective of ground water quality, “worst case” drought recharge conditions were used, where the 1961-1966 New Jersey drought of record was used to calibrate the GSR-32 model to extreme climatic conditions.
The final variable required for computing septic system density with Trela-Douglas is the annual load of nitrate mass generated per septic system, which in this analysis was assumed to be 40 pounds per year. This value was estimated on the basis of demographics for the Highlands Region (assumed average of 4 persons residing per dwelling with septic systems), and studies that indicate that a person generates approximately 10 pounds of nitrate annually. The 4 person average exceeds the 2.8 estimated mean household size for the Region, but was used to account for other possible nitrate loading sources, such as fertilizer applications to lawns, and for possible variations in household size within the Region.
After the input values for the input variables were obtained, the Trela-Douglas model was used to calculate acceptable septic system densities for the three LUC Zones in the Planning Area. However, these densities are expressed as average lot sizes for ease of calculation, and should not be interpreted as minimum lot sizes in zoning. For the 183 subwatersheds, the median septic system densities computed for the Existing Community, Conservation, and Protection Zones are 9.4, 10.0 and 26.1 acres per septic system, respectively. It should be noted that a number of these subwatershed are located exclusively in the Preservation Area, and would not be subject to these Planning Area densities. The septic system densities in the Highland’s Preservation Area, as computed by NJDEP using Trela-Douglas, are 25 and 88 acres in the non-forested and forested areas, respectively. These Preservation area densities were computed with target nitrate concentrations of 0.21 and 0.76 mg/L for the forested and non-forested areas, respectively.
Following computation of the appropriate densities with the Trela-Douglas model, septic system yields (i.e., a calculation of the additional allowable septic systems within a specific area) were computed within the three Planning Area Zones for each municipality based upon developable land existing within each. Developable land was estimated from existing MODIV (tax assessment) data, and included both vacant and oversized lots, as defined by septic system densities, and excluded publicly owned lands. The septic system yield was then computed for each municipality by dividing calculated septic system density into the developable planning land area available for each Zone.
Highlands Water Resources Technical Report Volume 1: Watersheds and Water Quality
119
The total combined number of additional allowable septic units within the Protection and Conservation Zones of the Planning Area is 6,544.
However, in the Preservation Area, septic system yield will be implemented by NJDEP based on individual projects. Many lots have a mix of forested and non-forested lands, and therefore the total calculated septic system yield for the Preservation Area will likely be somewhat smaller due to the need for development yields of individual parcels to be rounded down. Additionally, the existence of exempt lots makes a total calculation of septic system yield problematic. Therefore, septic yields for the Preservation Area are not included in this report.
This document is organized as follows: first, the general policy issues and background which served as the basis for developing the water quality targets for various areas within the Highlands Region, as well as a brief technical overview of the modeling methodology used for computing appropriate septic system densities and corresponding well yields, are presented. Second, the well data, methodology, and results for computing the baseline regional median nitrate concentration are presented. Third, the logistic regression modeling methodology and associated data used for computing median nitrate concentrations at the subwatershed scale and regional scale, as well as the possible utilization of these models for estimating appropriate septic system densities, are presented. Fourth, the Trela-Douglas nitrate dilution model for estimating appropriate septic system densities is presented, with some discussion given to logistic regression-based methods that were also considered, are presented and finally, the methodology for computing the number of additional allowable septic systems per municipality per zone with the results, are presented.
HIGHLANDS COUNCIL APPROACH TO NITRATE BACKGROUND CONCENTRATION AND SEPTIC SYSTEM YIELD