A Geochemical Study of the Effects of Land Use on
Final report on Research on this Grant Funded by the Suffolk County Water Authority
Gilbert N. Hanson
Department of Geosciences
Summary of results of studies
There were two aspects the studies that we have undertaken. The first was to characterize the sources of nitrate in the Northport area using nitrogen and oxygen isotopes in groundwater nitrate. This project is completed and has been reported in Bleifuss (1998). The second was to begin studies to understand the fate of ammonium in Long Island's sediments. This project is underway and we are planning to continue these studies.
The results reported here are a summary from Bleifuss (1998). (The results of Bleifuss (1998) can be found on the web at this link.) Nitrate concentrations in many of the wells in the vicinity of Northport, Long Island either approach or exceed the U. S. drinking water standard of 10 mg/l nitrogen as nitrate (Freeze and Cherry, 1979). The Suffolk County Water Authority monitors these wells on a weekly basis and combines water from several wells in order to ensure that the water being provided to the public meets drinking water standards. The study area (Fig. 1) is located along the north shore of Long Island between Northport Bay and Sunken Meadow State Park and extends south to the groundwater divide which is coincident with the Ronkonkoma Moraine.
Potential sources of nitrate in the Northport area included:
At present, there are only a few agricultural pockets in the area. In the past, the upgradient area, between the Harbor Hill and the Ronkonkoma moraines, was extensively farmed. Disposal of sewage is primarily through individual septic systems as public sewers are not provided in most of the area or in upgradient communities (personal communication, Suffolk County Department of Public Works) with the exception of a small network of sewers located in the center of Northport (Harland Bartholemew and Associates, 1962).
The objective of this study was to place constraints on the relative contributions of these various sources to nitrate contamination of the aquifer. The approach was to use the isotopes of nitrogen and oxygen in the nitrates in the water and dissolved major element ratios to characterize the sources of nitrates in the water. Water samples for analysis were collected from both monitoring wells and municipal supply wells. Monitoring wells are 2-6 inches in diameter and have screens that are 10 feet or less in length so they provide depth specific samples. Wells were selected for sampling that are down gradient from a particular source. Samples from these wells are used to place constraints on the nitrogen and oxygen isotopic composition of end-member nitrate sources.
Due to the heterogeneity of inputs to the Upper Glacial Aquifer, samples obtained from monitoring wells are not representative of conditions within the aquifer as a whole. To better characterize the aquifer representative samples were obtained from the public supply wells. These wells are generally 10 to 16 inches in diameter and are screened over 20-60 foot intervals. Samples from these wells provide depth integrated samples that are representative of the average concentration and isotopic composition of nitrates present within a cross section of the aquifer.
Numerous researchers have successfully used nitrogen isotopes to characterize nitrate sources and also to identify processes such as denitrification that may alter the concentration of nitrate within the aquifer system (Mariotti, et al., 1988). Several recent studies have taken advantage of the additional constraint that can be provided by the measurement of the oxygen isotopic composition of the nitrate (Fig. 2). There is a large oxygen isotopic contrast between nitrates produced in the atmosphere and those produced by microbial processes in the soil (nitrification). As a result the oxygen isotopes in nitrate are particularly useful for separating fertilizer nitrates (Amberger and Schmidt, 1987) and atmospheric nitrates (Durka, et al., 1994) from nitrates derived from ammonium fertilizer, septic systems and animal wastes.
Fig. 4Plot of nitrogen isotope ratios in nitrate in groundwaters from municipal supply wells in the Northport area at the top and monitoring wells at the bottom. The black bar represent the range in values. The positions of the location sites are at the nitrogen isotopes values for these areas.
Nitrogen and Oxygen Isotopic Compositions
The results of these studies (Fig. 3 and 4) show that:
d15N value compared to turf and agricultural areas
the deeper public supply wells which contain older water are showing a larger agricultural or low density residential component than do the monitoring wells.
Major Element Compositions
Ternary plots of the major cations and the major anions present in ground water (constructed from water quality data in the Land Use Monitoring Study, Suffolk County Comprehensive Water Resources Management Plan, 1988) illustrate that the major ion composition of ground water varies depending on the associated land use activity. For example, agricultural waters contain elevated proportions of calcium, magnesium and sulfate while septic plumes are high in bicarbonate, sodium and potassium (Fig. 5).
The major element data for Northport show that:
Sources of Nitrate in the Public Supply Wells
The nitrogen isotopic compositions of the public supply wells are consistent with low to medium density residential development. The data are also consistent with mixing with older agricultural water and younger residential water. This is because while the isotopic composition of nitrogen in nitrate from agricultural areas or areas dominated by turf fertilizer are somewhat different, they are both less than that for septic tank effluent.
It seems unlikely, however, that the nitrate contamination of public supply wells is primarily due to previous agricultural practices. This is because the major elements clearly show a residential signature. Samples from monitoring wells in residential areas show a positive correlation between sodium concentration andd15N (Fig. 6). This same correlation is seen for water from public supply wells. Therefore the heavier nitrogen in the public supply wells is probably associated with septic tank effluent.
The high concentration of nitrate in the public supply wells in the Northport area is surprising given the relatively low population density. The high concentration of nitrate probably results because of several factors:
On average 755 gallons per acre per day are withdrawn compared to the natural recharge of 1720 gallons per acre per day (SCDHS, 1987; Peterson, 1987). This means that 30% of the recharge consists of previously contaminated water. Nitrogen loading calculations suggest that more than 50% of the nitrate in the groundwater may be from turf grass fertilization. The nitrogen isotope composition of nitrates collected from residential monitoring wells is consistent with a similar percentage.
Although previous agricultural practices may contribute nitrate to water in some of the deeper wells, the nitrate contamination problem cannot be expected to disappear as this older water moves out of the system. Nitrate in some of the shallower well fields can be linked to residential land use. The closure of the shallower wells in these fields clearly demonstrates that changes in residential land use practices are necessary to protect the quality of the water in the Northport area.
Fate of Ammonium
The nitrate in the groundwater in the Northport area whether from agricultural or residential land use is the result of the nitrification of ammonium. It is quite clear that if we are to understand how nitrate gets into the groundwater we need to better understand the processes involved in the transport and nitrification of ammonium along its flow path. This is essential for determining the potential concentration of nitrate in groundwater from a given source and for evaluating the sources of nitrate in a given groundwater. Processes that affect ammonium include ammonia volatilization, nitrification of ammonium and retardation. Retardation is controlled by the cation exchange capacity of the material the ammonium passes through. During transport in aqueous solution ammonium ions may be preferentially sorbed on the coatings of grains in sediments. This is because ammonium is sorbed more readily than the major cations in solution (Mg, Ca, and Na) excepting K. The main emphasis of our studies to date are to:
Ms. Boguslavsky1 has found relatively high exchange capacities associated with the iron-rich coatings on the dominantly quartz sand grains characteristic of Long Island (about 20 milliequivalents per 100 grams of sediment). Organic matter and clays as well as iron-oxides form the coatings. She is also determining the mineralogical composition, chemical composition, total organic carbon, the cation exchange capacity and organic distribution coefficient of a range of sediments from different levels below the surface. Once a series of these data are available it should be possible to model more precisely how the composition of a fluid will change as it passes through the unsaturated zone sediments.
While fertilizers may be as important as a source of nitrate in groundwater, it is our impression that septic tank effluent may be easier to characterize and model. Surprisingly there is very little major element, much less trace element, data for septic tank - cess pool effluent from Long Island residences. Chris Biemiller of Suffolk County Department of Public Works is beginning to collect such data. He has expressed a willingness to cooperate with us on the use of such data.
The modeling should allow us to evaluate what happens to the ammonium as it leaves the cess pool and how the effluent changes composition along the flow path to the ground water. The chemical signature of the cess pool effluent is expected to be significantly different and to show a larger range of compositions as a result of exchange interactions of the fluid with the sediment in the unsaturated zone. This modeling should allow us to find tracers that will allow us to better determine the relative input of nitrogen from fertilizers and septic tank - cess pool effluent.
When effluent from a cess pool enters an aerobic environment ammonium undegoes nitrification to nitrate. However, around the fringes of a cess pool there is an anaerobic environment in which ammonium remains stable. In 30 years a family of four produces about 2.6 x 105 meq of nitrogen in their sewage. Just considering a fringe volume six inches wide along the bottom and four feet up the sides of a typical cess pool (Fig. 6) we calculate that this volume will easily accommodate one-half of this nitrogen as ammonium produced by the family over a period of 30 years. This possible retention of ammonium around the cess pool may explain why about one-half of the nitrogen from sewage systems is unaccounted for. It might also explain why Katz et al, 1980 found no significant difference in the median nitrate content of groundwater in sewered and unsewered areas of Nassau County. This may be because after an area has been sewered the ammonium surrounding the cess pool will continually undergo nitrification and enter the groundwater as nitrate.