Evaluation of Nitrate Concentrations and Potential Sources of Nitrate in Private Water Supply Wells in North Carolina.
Nitrogen and Environmental Health
Nitrogen is a crucial element for the development of proteins and other organic substances that directly influence plant and animal life. Insufficient concentrations of plant-available forms of nitrogen, such as nitrate, can limit crop yields and primary production in terrestrial and aquatic environments (Conley et al., 2009; Havlin, Beaton, Tisdale, & Nelson, 1999). Elevated nitrate concentrations in water resources, however, can be detrimental to public and environmental health. For example, research has suggested that prolonged consumption of high concentrations of nitrate in drinking water can increase the risk of cancer, methemoglobinemia (blue baby syndrome), and cumulative dysfunctions in organ systems (Ward et al., 2005).
The maximum contaminant level for nitrate-nitrogen (N[O.sub.3]-N) in drinking water supplies across the U.S. (and North Carolina) is 10,000 [micro]g/L (U.S. Environmental Protection Agency [U.S. EPA], 2009), but concentrations much lower can cause environmental health concerns. For example, concentrations of nitrate-nitrogen that exceed 1,000 [micro]g/L can stimulate hypergrowth of algae in some surface waters, leading to eutrophication (Osmond et al., 2003). Some algae produce toxins such as microcystins that are hazardous to humans and animals, and thus are environmental health threats (North Carolina Department of Environmental Quality, 2015; Smith & Schindler, 2009). Algal blooms can impact drinking water supplies by clogging water filters and imparting undesirable tastes and odors (Dodds et al., 2009). When the algae eventually die and decompose, surface waters can become depleted of dissolved oxygen, leading to fish kills and water use impairment (Conley et al., 2009).
Major water resources in North Carolina including the Neuse River, Tar-Pamlico River, Falls Lake, and Jordan Lake are nutrient sensitive and watershed nutrient management rules have been or are being developed to reduce nitrogen loadings to these waters (North Carolina Department of Environmental Quality, n.d.). Identifying the main contributors of nitrate to water resources is important for policy development that will help protect public and environmental health.
Agricultural Nitrate Sources
There are many different sources of nitrate-nitrogen in the environment including fertilizer, animal waste, human waste, atmospheric deposition, and nitrogen fixation. Nitrogen fertilizer production and application to agriculture fields has increased four-fold since the 1960s (Havlin et al., 1999). The increased nitrogen applications not only led to an increase in crop yields and overall agricultural production but also raised concerns for environmental contamination resulting from nitrate that is not used by the crops (Havlin et al., 1999). Industrial livestock farms, also called confined animal feeding operations (CAFOs), produce animal wastes with elevated concentrations of nitrogen (>400,000 [micro]g/L) that are often spray irrigated, or dried and applied onto crops (Goldberg, 1989; Huang, Yang, & Ling, 2014). Research has shown that nitrate concentrations in groundwater and surface waters near CAFOs and agricultural fields can exceed water-quality standards (Stone, Hunt, Humenik, & Johnson, 1998).
Agriculture is a major industry in North Carolina where farm receipts have totaled over 10 billion/year since 2010 (North Carolina Department of Agriculture and Consumer Services, 2016). There are more than 8 million hogs, 800 million poultry, 800,000 cattle, 1.9 million hectares (ha) of cropland, and 0.46 million ha of pasture in the state (North Carolina Department of Agriculture and Consumer Services, 2016). Commercial fertilizer (such as urea and ammonium nitrate) and manure are applied to approximately 72% and 10% of the cropland, respectively, in North Carolina (North Carolina Department of Agriculture and Consumer Services, 2016; Osmond et al., 2003).
Corn is one of the most commonly grown crops in North Carolina (>325,000 hectares) with nitrogen application rates typically exceeding 136 kg/ha (North Carolina Department of Agriculture and Consumer Services, 2016; North Carolina State Extension, 2017; Osmond et al. 2003). Crop uptake of nitrogen is relatively inefficient (~50%), leading to nitrogen loss via leaching, volatilization, and/ or denitrification (Osmond & Kang, 2008). Therefore, groundwater quality can be influenced by nitrate leaching from agricultural lands receiving fertilizer and manure, especially in well drained, sandy regions of the state (Osmond et al., 2003; Stone et al., 1998).
Septic Systems and Nitrate
Septic systems, or onsite wastewater treatment systems (OWTS), are another potential source of nitrate in groundwater (Del Rosario, Humphrey, Mitra, & O'Driscoll, 2014; Humphrey, O'Driscoll, & Zarate, 2010; Humphrey et al., 2013). OWTS are commonly used for treating wastewater in rural areas of North Carolina and other states (U.S. EPA, 2002). OWTS include a septic tank, conveyance pipes, drainfield trenches, and aerated soil under the drainfield trenches (Humphrey et al., 2013). Septic tank effluent has concentrations of ammonium-nitrogen that often exceed 35,000 [micro]g/L, and the ammonium can be quickly converted to nitrate-nitrogen in aerated soils beneath OWTS drainfield trenches via the nitrification process (Humphrey et al., 2013).
Nitrate concentrations exceeding the maximum contaminant level of 10,000 [micro]g/L have been reported in groundwater near OWTS in numerous studies conducted in North Carolina (Del Rosario et al., 2014; Humphrey et al., 2010; Humphrey et al., 2013; Iverson, O'Driscoll, Humphrey, Manda, & Anderson-Evans, 2015). An estimated 50% of residents in North Carolina use OWTS (Pradhan, Hoover, Austin, & Devine, 2007), and thus OWTS might also be a significant source of nitrate in groundwater.
Atmospheric deposition of nitrogen can occur via precipitation (wet deposition) or during movement and settling of aerosol particles by wind (dry deposition) (Gao, Kennish, & McGuirk Flynn, 2007). North Carolina receives on average 100-140 cm of rain in the Piedmont and Coastal Plain regions, and 94-229 cm in the mountains (State Climate Office of North Carolina, 2017). Regions downwind or close to industrialized areas or CAFOs are more likely to show an increased nitrogen load due to atmospheric deposition (Whitall & Paerl, 2001). Nitrogen in the atmosphere as [N.sub.2] gas can be fixed by some terrestrial and aquatic organisms to plant-available forms such as ammonium and converted to organic nitrogen (Havlin et al., 1999). Fixation can also occur via lightning strikes (Meyer, 2014). When the nitrogen-fixing plants and organisms die and decompose, the organic nitrogen in the cells can be mineralized, converted to ammonium and nitrate, and released into the environment (Havlin et al., 1999).
Groundwater Supplies and Nitrate
There are approximately 3 million people (31% of the population) who use groundwater for a water supply in North Carolina (North Carolina Department of Health and Human Services, 2016). The installation of private wells is permitted and regulated at the local level by county health departments. New wells must meet certain criteria for structural standards and setback distances, and are sampled and tested for contamination prior to initial use. The state of North Carolina performs water testing for contaminants such as nitrate for new and existing wells upon request. A database of sample results is kept on file by the state.
Study Goal and Objectives
The goal of this research was to gain a better understanding of potential links between land use and nitrate concentrations in well water across North Carolina. The research objectives included the following: 1) to determine if there are statistically significant correlations between the percentage of land in agriculture, livestock numbers and densities, septic systems, and average groundwater nitrate concentrations in North Carolina counties; 2) to determine the 10 counties in North Carolina with the highest and the 10 counties with the lowest average concentrations of nitrate in groundwater and summarize their associated land-use characteristics; and 3) to determine if there are statistically significant differences in livestock numbers, livestock densities, septic system numbers and densities, and land area in agriculture for the 10 counties with highest average nitrate concentrations in groundwater in comparison with the 10 counties with the lowest average nitrate concentration in groundwater. These analyses were performed to determine which land-use characteristics were strongly associated with relatively high mean concentrations of nitrate in groundwater.
Groundwater Nitrate Concentrations and Land-Use Characteristics Determination
Groundwater nitrate concentration data (1998-2010) from drinking water wells in North Carolina were obtained from the North Carolina State Laboratory of Public Health. More than 31,000 samples were analyzed during the time frame. The groundwater nitrate data from drinking water wells were organized in spreadsheets and the average nitrate concentration in groundwater samples was calculated for each county (North Carolina Department of Health and Human Services, 2016).
Agricultural land-use characteristics including the number of cattle, poultry, hogs, and cropland acreage were summarized for each county using published data from the North Carolina Department of Agriculture and Consumer Services (2016). The numbers of livestock and acreage in cropland were divided by the total land area for each county to determine the density of livestock and fraction of land in crop production.
The two latest U.S. Census Bureau reports (2000, 2010) did not include information with regards to use of septic systems. Septic system data from the 1990 U.S. Census Bureau and North Carolina Environmental Health Reports were analyzed to obtain a more current number of septic systems used in each county and in the state. The Environmental Health Division of the North Carolina Department of Health and Human Services collects yearly information regarding all onsite wastewater activities including the number of new septic system permits issued (North Carolina Department of Health and Human Services, 2018).
We used the number of operation permits (OPs) issued each year as the unit of measurement for new system installations in each county. The number of OPs issued each year was added to the number of septic systems reported in the 1990 U.S. Census to gain a more current estimate of the number of systems in each county and the state. The average number of people per dwelling as indicated by the 2010 Census was multiplied by the number of systems in each county and divided by the total population to determine the percentage of people using septic systems in each county. The average percentage of population using septic systems was calculated for all 100 North Carolina counties. The number and density of septic systems used in each county were calculated. Septic system density was calculated by dividing the total land area (ha) of a county by the total number of septic systems.
The average concentrations of nitrate in groundwater wells were compared to the numbers and densities of potential nitrate sources to determine if there were statistically significant correlations and to provide insight into the most significant sources of nitrate in groundwater. Nitrate and land-use data were organized by county, and all 100 counties in North Carolina were included in the correlation analyses. More specifically, the number of active septic systems in each county and the density of septic systems; the number and density of hogs, poultry, cattle, and all livestock; and the fraction of total land in agriculture were each compared to the average nitrate concentrations. Spearman correlation analyses were performed with Mini tab 17 statistical software to determine which land-use factors were significantly correlated with nitrate concentrations. Summary tables were developed listing the correlation coefficients and p-values for the comparisons.
There are 100 counties in North Carolina. Characteristics of the 10 counties with the highest average nitrate concentrations (top 10%) were summarized and compared with the 10 counties with the lowest average nitrate concentrations (bottom 10%) to determine if significant differences in numbers and densities of livestock, septic systems, and cropland were observed. It was anticipated that differences in the major land-use characteristics (numbers and densities) that influence groundwater nitrate concentrations would be significant when comparing the counties with the highest and lowest average nitrate concentrations. Comparisons were made using paired t-tests or Mann-Whitney tests (for data that did not follow a normal distribution) to determine if differences in numbers and densities of nitrate sources for the top 10 and bottom 10 counties were statistically significant (p < .05). These analyses were conducted to provide insight into the major factors associated with elevated concentrations of nitrate in groundwater.
Correlations Between Average Nitrate Concentrations and Land-Use Characteristics
There were statistically significant (p < .05) correlations between the average nitrate concentrations in groundwater and various county land-use characteristics including farmland acreage (p < .001), total livestock (p < .001), human population (p = .003), number of poultry (p = .001), number of cattle (p = .007), number of septic systems (p = .038), and number of hogs (p = .049) (Table 1). The correlation coefficients were greatest for farmland acreage (r = .456), total livestock (r = .396), and poultry (r = .331) (Table 1). While there was a statistically significant correlation between the number of septic systems and average nitrate concentrations, the correlation coefficient was the smallest (r = .209) of the potential sources (Table 1).
There were statistically significant correlations between average nitrate concentrations and fraction of land in agriculture (p < .001), and densities of total livestock (p < .001), people (p = .002), poultry (p = .001), hogs (p < .001), cattle (p = .004), and septic systems (p = .047) (Table 2). The correlation coefficients (average nitrate concentration and density of potential sources) were greatest for fraction of land in agriculture (r = .486), hog density (r = .391), total livestock (r = .382), and poultry density (r = .328) (Table 2). Correlation coefficients were smallest for density of septic systems (r = .200) and density of cattle (r = .290) (Table 2).
Counties With Highest and Lowest Average Nitrate Concentrations in Groundwater
The 10 counties with the highest average concentrations of nitrate in groundwater included Cumberland, Edgecombe, Greene, Halifax, Northampton, Richmond, Robeson, Sampson, Wake, and Wayne (Table 3). The overall average groundwater nitrate concentration for these 10 counties was 3,429 [micro]g/L, with a range of 3,003-4,504 [micro]g/L (Table 3). The top 10 counties are clustered in the inner Coastal Plain and Sand Hills region of the state (Figure 1).
The 10 counties with the lowest average nitrate concentrations in groundwater were Beaufort, Camden, Chowan, Clay, Gates, Hyde, Jackson, Macon, Pasquotank, and Perquimans (Table 4). The overall average nitrate concentration for these counties was 510 [micro]g/L, with a range of 500-533 [micro]g/L (Table 4). Seven of these counties are clustered in the Tidewater region of the state, and the other three are clustered in the mountains (Figure 1). The difference in nitrate concentrations between the top 10 and bottom 10 counties was statistically significant (p = .0001).
There were more people (174,199 versus 22,943), septic systems (25,151 versus 9,101), hogs (338,500 versus 8,400), poultry (17,192,710 versus 2,108,540), and cattle (6,430 versus 1,111) in the 10 counties with the highest average nitrate concentrations relative to the 10 counties with the lowest nitrate concentrations (Tables 3 and 4). There was also more land area on average for the top 10 counties (168,551 ha) relative to the bottom 10 counties (130,772 ha), so we also compared densities (numbers/area) of potential sources of nitrate including septic systems, livestock, and fraction of the overall county land that was in agriculture. Similar findings were observed when normalizing the data for land area.
More specifically, there was a higher average fraction of land in agriculture (0.38 versus 0.23) and higher densities of septic systems (0.14/ha versus 0.08/ha), people (0.91/ ha versus 0.22/ha), hogs (1.92/ha versus 0.06/ha), poultry (99.4/ha versus 25.6/ha), and total livestock (101.5/ha versus 25.7/ ha) in the 10 counties with the highest average nitrate concentration (Tables 5 and 6). There were statistically significant differences regarding the fractions of land in agriculture (p = .036), along with density of hogs (p = .0049), poultry (p = .0299), cattle (p = .0257), total livestock (p = .0211), and people (p = .0140) when comparing the top 10 with the bottom 10. While the mean density of septic systems was higher in the top 10 versus bottom 10, the differences were not statistically significant (p = .1041).
Agriculture and Groundwater Nitrate Concentrations
There are many potential sources of nitrate in groundwater sampled from water supply wells across North Carolina. Most of the data indicate that agricultural sources such as fertilizers and total livestock (waste) might be the most important contributors of nitrate to groundwater in North Carolina. There was on average 15% more land in agriculture and 75 more livestock/ha in the 10 counties with the highest average nitrate concentrations relative to the 10 counties with the lowest average nitrate concentrations. There were statistically significant (p [less than or equal to] .05) correlations between average nitrate concentrations and fraction of land in agriculture and total livestock densities; and the correlation coefficients were greatest for average nitrate concentration and fraction of land in agriculture, hog density, and total livestock density.
Four of the top 10 counties in North Carolina for hog production were in the top 10 for highest mean nitrate concentrations in groundwater. Those counties included Greene, Robeson, Sampson, and Wayne, which are all located in the Inner Coastal Plain of North Carolina. The total combined number of hogs produced each year by these counties was 2,970,000, which is nearly 40% of all hogs produced in the state (North Carolina Department of Agriculture and Consumer Services, 2016). Prior studies have shown that hog farms can be significant contributors of nitrogen to shallow groundwater (Stone et al., 1998) and surface waters (Mallin, McIver, Robuck, & Dickens, 2015).
The 10 counties with the highest mean concentrations of nitrate were all located in the Inner Coastal Plain or Sand Hills region, where the soils are characterized as permeable, well drained, and prone to nitrate leaching (Gilliam et al., 1996). Row crop and livestock production is extensive in the Inner Coastal Plain. Seven of the 10 counties with the lowest mean nitrate concentrations were in the Tidewater region of the state where row crop and livestock production are not as intensive as the Inner Coastal Plain and the soils have a high organic matter content, are poorly drained, and denitrification potential is high due to these conditions (Havlin et al., 1999).
Septic Systems and Groundwater Nitrate Concentrations
An estimated 4.87 million people in North Carolina were using septic systems in 2010, which was approximately 50% of the total population during that year. The percentage of population using septic systems varied greatly from county to county with a range of 10% to >90%. The last time the U.S. Census Bureau included information on septic system usage, 49% of the population in North Carolina used septic systems, so statewide, the percentage using septic systems has remained steady since 1990. While there were more septic systems and higher densities of septic systems in the 10 counties with the highest average nitrate concentrations, the differences were not statistically significant (p > .05). The correlations between average nitrate concentration and total number of septic systems and density of septic systems in North Carolina counties were significant, but they had the lowest correlation coefficients (r < .021) of the potential sources. Therefore, there was some evidence that septic systems were a contributing source of nitrate to groundwater, but the contributions were not as significant as agriculture.
The goal of this study was to gain a better understanding of how various land uses and nitrate sources might influence groundwater nitrate concentrations in North Carolina. Counties with extensive agricultural production located in geological settings where nitrate leaching potential is great, such as the Inner Coastal Plain, had the highest average concentrations of nitrate in groundwater. Counties with poorly drained, organic soils and less intensive agricultural and livestock production had the lowest average concentrations of nitrate in groundwater. Agriculture is a vital industry in North Carolina for the state's economy and for food production. Continued, substantial funding for the development and implementation of best management practices to reduce nitrogen loss from agricultural fields--especially in the Inner Coastal Plain of North Carolina--is needed to ensure a balance between the environment and economy.
Corresponding Author: Charles Humphrey, Associate Professor, Environmental Health Sciences Program, East Carolina University, 3408 Carol Belk, Greenville, NC 27858. E-mail: email@example.com.
Conley, D.J., Paerl, H.W., Howarth, R.W., Boesch, D.F, Seitzinger, S.P., Havens, K.E., ... Likens, G.E. (2009). Controlling eutrophication: Nitrogen and phosphorus. Science, 323(5917), 1014-1015.
Del Rosario, K.L., Humphrey, C.P., Mitra, S., & O'Driscoll, M.A. (2014). Nitrogen and carbon dynamics beneath on-site wastewater treatment systems in Pitt County, North Carolina. Journal of Water Science and Technology, 69(3), 663-671.
Dodds, W.K., Bouska, W.W., Eitzmann, J.L., Pilger, TJ., Pitts, K.L., Riley, A.J., ... Thornbrugh, D.J. (2009). Eutrophication of U.S. freshwaters: Analysis of potential economic damages. Environmental Science & Technology, 43(1), 12-19.
Gao, Y., Kennish, M.J., & McGuirk Flynn, A. (2007). Atmospheric nitrogen deposition to the New Jersey Coastal Waters and its implications. Ecological Applications, 17(5), S31-S41.
Gilliam, J.W., Huffman, R.L., Daniels, R.B., Buffington, D.E., Morey, A.E., & Leclerc, S.A. (1996). Contamination of surficial aquifers with nitrogen applied to agricultural land (WRRI Project No. 70114). Retrieved from https://repository.lib.ncsu.edu/bitstream/handle/ 1840.4/1868/NC-WRRI-306.pdf?sequence=1&isAllowed=y
Goldberg, VM. (1989). Groundwater pollution by nitrates from livestock wastes. Environmental Health Perspectives, 83, 25-29.
Havlin, J.L., Beaton, J.D., Tisdale, S.L., & Nelson, W.R. (1999). Soil fertility and fertilizers: An introduction to nutrient management (6th ed., pp. 1-7, 86-153). Upper Saddle River, NJ: Prentice Hall.
Huang, H., Yang, J., & Ling, D. (2014). Recovery and removal of ammonia-nitrogen and phosphate from swine wastewater by internal recycling of struvite chlorination product. Bioresource Technology, 172, 253-259.
Humphrey, C.P., Jr., O'Driscoll, M.A., Deal, N.E., Lindbo, D.L., Thieme, S.C., & Zarate-Bermudez, M.A. (2013). Onsite wastewater system nitrogen contributions to groundwater in Coastal North Carolina. Journal of Environmental Health, 76(5), 16-22.
Humphrey, C.P., Jr., O'Driscoll, M.A., & Zarate, M.A. (2010). Controls on groundwater nitrogen contributions from on-site wastewater systems in coastal North Carolina. Water Science and Technology, 62(6), 1448-1455.
Iverson, G., O'Driscoll, M.A., Humphrey, C.P., Jr., Manda, A.K., & Anderson-Evans, E. (2015). Wastewater nitrogen contributions to Coastal Plain watersheds, NC, USA. Water, Air, and Soil Pollution, 226(10), 325.
Mallin, M.A., McIver, M.R., Robuck, A.R., & Dickens, A.K. (2015). Industrial swine and poultry production causes chronic nutrient and fecal microbial stream pollution. Water, Air, and Soil Pollution, 226, 407.
Meyer, R.F (2014, April 17). Understanding nitrates in our water. The Burlington Record. Retrieved from http://www.burlington-record. com/letters/ci_25585509/understanding-nitrates-our-water
North Carolina Department of Agriculture and Consumer Services. (2016). Agricultural statistics. Retrieved from http://www.ncagr. gov/stats/index.htm
North Carolina Department of Environmental Quality, (n.d.). Nutrient sensitive waters and special watersheds. Retrieved from https:// deq.nc.gov/about/divisions/energy-mineral-land-resources/ nsw-special-watersheds
North Carolina Department of Environmental Quality. (2015). Algal blooms, fish kills occurring across portions of the state. Retrieved from https://deq.nc.gov/press-release/algal-blooms-fish-killsoccurring-across-portions-state
North Carolina Department of Health and Human Services. (2016). Well water and health. Retrieved from http://epi.publichealth. nc.gov/oee/wellwater/figures.html
North Carolina Department of Health and Human Services. (2018). On-site water protection branch: On-site wastewater treatment and dispersal systems program resources. Retrieved from http://ehs. ncpublichealth.com/oswp/resources.htm
North Carolina State Extension, College of Agriculture and Life Sciences, North Carolina State University. (2017). 2018 North Carolina agricultural chemicals manual. Retrieved from https://content. ces.ncsu.edu/north-carolina-agricultural-chemicals-manual
Osmond, D.L., Hodges, S.C., Kleiss, H.J., Creamer, N.G., Crozier, C.R., Cubeta, D.H., ... Weisz, R. (2003). Tar-Pamlico River Basin nutrient management education [slide set]. Raleigh, NC: North Carolina Cooperative Extension Service, North Carolina State University.
Osmond, D.L., & Kang, J. (2008). Nutrient removal by crops in North Carolina (AG-439-16W). Soil facts. Retrieved from https://con tent.ces.ncsu.edu/nutrient-removal-by-crops-in-north-carolina
Pradhan, S.S., Hoover, M.T., Austin, R.E., & Devine, H.A. (2007). Potential nitrogen contributions from on-site wastewater treatment systems to North Carolina's river basins and sub-basins. North Carolina Agricultural Research Service Technical Bulletin, 324. Retrieved from www.soil.ncsu.edu/publications/TB324Final may29.pdf
Smith, VH., & Schindler, D.W. (2009). Eutrophication science: Where do we go from here? Trends in Ecology & Evolution, 24(4), 201-207.
State Climate Office of North Carolina. (2017). North Carolina climate office: General synopsis. Retrieved from http://climate.ncsu. edu/climate/synopsis
Stone, K.C., Hunt, P.G., Humenik, FJ., & Johnson, M.H. (1998). Impact of swine waste application on ground and stream water quality in an Eastern Coastal Plain watershed. Transactions of the American Society of Agricultural Engineers (ASAE), 41(6), 1665-1670.
U.S. Census Bureau. (1990). 1990 census data. Retrieved from http:// www.census.gov/main/www/cen1990.html
U.S. Census Bureau. (2010). 2010 census data. Retrieved from http:// www.census.gov/2010census/data
U.S. Environmental Protection Agency. (2002). Onsite wastewater treatment systems manual (EPA/625/R-00/008). Washington, DC: Office of Water, Office of Research and Development. Retrieved from https:// nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=30004GXI.TXT
U.S. Environmental Protection Agency. (2009). National primary drinking water regulations (EPA-816-F-09-004). Washington, DC: Author. Retrieved from http://www.nrc.gov/docs/ML1307/ ML13078A040.pdf
Ward, M.H., deKok, T.M., Levallois, P, Brender, J., Gulis, G., Nolan, B.T., & VanDerslice, J. (2005). Workgroup report: Drinking-water nitrate and health--Recent findings and research needs. Environmental Health Perspectives, 113(11), 1607-1614.
Whitall, D.R., & Paerl, H.W. (2001). Spatiotemporal variability of wet atmospheric nitrogen deposition to the Neuse River Estuary, North Carolina. Journal of Environmental Quality, 30(5), 1508-1515.
Emily Naylor, MSEH, REHS
Stokes County Environmental Health
Charles Humphrey, PhD, REHS
Tim Kelley, PhD
Environmental Health Sciences Program
East Carolina University
Leslie Easter, REHS
Stokes County Environmental Health
Guy Iverson, MS
East Carolina University
Caption: FIGURE 1 Map of North Carolina Showing Counties With the Highest and Lowest Average Nitrate Concentrations
TABLE 1 Correlations Between Total Number of Potential Sources of Nitrate and Average Nitrate Concentrations in Groundwater in North Carolina Counties Total Number of Sources and Correlation p-Value Average Nitrate Concentrations Coefficient Farmland (ha) and average nitrate .456 <.001 Total livestock and average nitrate .396 <.001 Poultry and average nitrate .331 .001 Hogs and average nitrate .322 .049 Total people and average nitrate .300 .003 Cattle and average nitrate .276 .007 Septic systems and average nitrate .209 .038 ha = hectares. TABLE 2 Correlations Between Density of Potential Sources of Nitrate and Average Nitrate Concentrations in Groundwater in North Carolina Counties Density of Source and Average Nitrate Correlation p-Value Concentration Coefficient Fraction agriculture and average nitrate .486 <.001 Hogs/ha and average nitrate .391 <.001 Total livestock/ha and average nitrate .382 <.001 Poultry/ha and average nitrate .328 .001 People/ha and average nitrate .306 .002 Cattle/ha and average nitrate .290 .004 Septic systems/ha and average nitrate .200 .047 ha = hectares. TABLE 3 Characteristics of the 10 North Carolina Counties With the Highest Average Nitrate Concentrations in Groundwater County Total County 2010 Census Average Septic Area (ha) Population N[O.sub.3]-N Systems ([micro]g/L) Cumberland 170,494 319,431 3,003 48,233 Edgecombe 131,368 56,552 3,284 10,315 Greene 68,923 21,362 3,196 6,566 Halifax 189,409 54,691 3,214 13,101 Northampton 142,769 22,099 3,520 8,153 Richmond 124,372 46,639 3,397 14,374 Robeson 246,413 134,168 3,909 32,636 Sampson 245,377 63,431 3,134 22,979 Wake 222,057 900,993 4,504 64,106 Wayne 144,324 122,623 3,227 31,052 Average 168,551 174,199 3,439 25,151 SD 60,338 285,873 471 20,071 County Cattle Hog Poultry Total Estimates Estimates Estimates Livestock Cumberland 2,900 95,000 2,645,000 2,742,900 Edgecombe 2,000 110,000 6,650,000 676,200 Greene 1,400 340,000 4,750,000 5,091,400 Halifax 9,500 45,000 0 54,500 Northampton 900 115,000 9,200,000 9,315,900 Richmond 1,800 50,000 34,910,000 34,961,800 Robeson 8,000 350,000 48,586,900 48,944,900 Sampson 26,000 1,750,000 47,185,000 48,961,000 Wake 3,500 0 9,200 12,700 Wayne 8,300 530,000 17,991,000 18,529,300 Average 6,430 338,500 17,192,710 17,537,640 SD 8,004 551,567 20,406,235 20,759,407 ha = hectares. TABLE 4 Characteristics of the 10 North Carolina Counties With the Lowest Average Nitrate Concentrations in Groundwater County Total County 2010 Census Average Septic Area (ha) Population N[O.sub.3]-N Systems ([micro]g/L) Beaufort 248,485 47,759 517 19,206 Camden 79,287 9,980 500 3,391 Chowan 60,372 14,793 500 4,504 Clay 57,263 10,587 533 5,510 Gates 89,652 12,197 500 4,416 Hyde 368,972 5,810 500 3,819 Jackson 128,000 40,271 526 17,006 Macon 115,563 33,922 520 20,937 Pasquotank 74,883 40,661 500 7,563 Perquimans 85,247 13,453 500 4,658 Average 130,772 22,943 510 9,101 SD 105,136 16,343 14 7,257 County Cattle Hog Poultry Total Estimates Estimates Estimates Livestock Beaufort 1,200 50,000 0 51,200 Camden 100 0 0 100 Chowan 1,200 4,000 0 5,200 Clay 1,700 0 2,550,000 2,551,700 Gates 800 30,000 7,850,000 7,880,800 Hyde ND 0 180,000 180,000 Jackson 1,600 0 4,200 5,800 Macon 2,400 0 1,200 3,600 Pasquotank 300 0 0 300 Perquimans 700 0 10,500,000 10,500,700 Average 1,111 8,400 2,108,540 2,117,940 SD 758 18,138 2,638,829 2,645,344 ha = hectares; ND = no data, if data were not reported for a county. TABLE 5 Characteristics and Densities of Potential Nitrate Sources for the 10 North Carolina Counties With the Highest Average Nitrate Concentrations in Groundwater County Fraction Systems/ha People/ha Hogs/ha Agriculture Cumberland 0.20 0.28 1.87 0.56 Edgecombe 0.39 0.08 0.43 0 Greene 0.59 0.10 0.31 4.93 Halifax 0.42 0.07 0.29 0.24 Northampton 0.46 0.06 0.15 0.81 Richmond 0.15 0.12 0.37 0.40 Robeson 0.44 0.13 0.54 1.42 Sampson 0.48 0.09 0.26 7.13 Wake 0.15 0.29 4.06 0 Wayne 0.54 0.22 0.85 3.67 Average 0.38 0.14 0.91 1.92 SD 0.16 0.09 1.29 2.54 County Poultry/ha Cattle/ha Livestock/ha Cumberland 15.50 0.02 16.0 Edgecombe 50.60 0.01 51.5 Greene 68.90 0.02 73.9 Halifax <0.01 0.05 0.3 Northampton 64.40 0.01 65.3 Richmond 280.70 0.01 281.1 Robeson 197.20 0.03 198.6 Sampson 192.30 0.11 199.5 Wake 0.04 0.02 0.1 Wayne 124.70 0.06 128.4 Average 99.40 0.03 101.5 SD 101.40 0.03 102.2 ha = hectares. TABLE 6 Characteristics and Densities of Potential Nitrate Sources for the 10 North Carolina Counties With the Lowest Average Nitrate Concentrations in Groundwater County Fraction Systems/ha People/ha Hogs/ha Agriculture Beaufort 0.24 0.08 0.19 0.20 Camden 0.25 0.04 0.13 <0.01 Chowan 0.39 0.07 0.25 0.07 Clay 0.08 0.10 0.18 <0.01 Gates 0.29 0.05 0.14 0.33 Hyde 0.12 0.01 0.02 <0.01 Jackson 0.05 0.13 0.31 <0.01 Macon 0.08 0.18 0.29 <0.01 Pasquotank 0.39 0.10 0.54 <0.01 Perquimans 0.38 0.05 0.16 <0.01 Average 0.23 0.08 0.22 0.06 SD 0.13 0.05 0.15 0.12 County Poultry/ha Cattle/ha Livestock/ha Beaufort <0.01 0.01 0.21 Camden <0.01 <0.01 <0.01 Chowan <0.01 0.02 0.09 Clay 44.50 0.03 44.56 Gates 87.60 0.01 87.90 Hyde 0.50 <0.01 0.50 Jackson 0.03 0.01 0.05 Macon 0.01 0.02 0.03 Pasquotank <0.01 <0.01 <0.01 Perquimans 123.17 0.01 123.18 Average 25.60 0.01 25.70 SD 31.00 0.01 31.10 ha = hectares.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||ADVANCEMENT OF THE SCIENCE|
|Author:||Naylor, Emily; Humphrey, Charles; Easter, Leslie; Iverson, Guy|
|Publication:||Journal of Environmental Health|
|Date:||May 1, 2018|
|Previous Article:||JEH QUIZ.|
|Next Article:||A Review of Nontuberculous Mycobacteria Presence in Raw and Pasteurized Bovine Milk.|