Groundwater Research Group

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Xuan Xu

Geoscience Department
Stony Brook University
Email: xuxu@ic.sunysb.edu

Hello! I am currently a graduate student of Geosciences Department at Stony Brook University. I am working with Dr. Gilbert Hanson on septic tank pollution on Long Island as well as determining denitrification by dissolved gas, such as N2 /Ar, in Long Island ground water.

CV

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Education:
M.S. Stony Brook University , 2004~current. Major: Geosciences
M.S. Peking ( Beijing ) University, 2001~2004. Major: Geology
B.S. Peking ( Beijing ) University, 1997~2001. Major: Geology

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Employment:
Research Assistant, Geoscience Department, Stony Brook University  August, 2005~Current
Teaching Assistant, Geoscience Department, Stony Brook University  Fall 2004
Teaching Assistant, Geology Department, Peking ( Beijing ) University  Fall 2003

Research Interest:

My current project is focused on a septic tank plume from a residential house in southern Long Island . We are trying to trace the plume in the ground water system and understand how it is interacting with sediments.

Long Island Ground water system:

Ground water is the sole source of fresh drinking water for more than 2.7 million residents of Nassau County and Suffolk County . Acid rain, fertilizer usage on lawn, agriculture and pollution from sewage are major concerns about chemical quality of Long Island ground water. Increased nitrate concentration in Long Island ’s ground water has potential health hazard to infants and may result in chronic toxicity and possible development of cancer.

Suffolk County has the highest concentration of cesspools in New York State . Most of these cesspools are built before 1972 when a separate septic tank system was not required. The open bottom of the cesspool is leaching the sewage directly into the vadose zone through which it may end up into the ground water system. High concentration of nitrate in open water systems and the Upper Glacial aquifer is contributed mainly to the widely used septic tank, so that knowledge of the interaction of sewage plume and sediment in Long Island is essential in water-management decision.

Septic Tank System:

Figure 1 is a typical septic tank system used in Long Island . Regulation proposed the minimum distance from a pumping well to the septic tank and leaching pool as 75ft and 100 ft respectively. This minimum distance is made based on an assumption that the sediment will take up the nutrients, such as metal and the nitrate, in this distance so that the sewage from the cesspool will not affect the water quality pumped from the supplying well. But, as a matter of fact, this assumption has rarely been tested!

My study is concentrated on a sewage plume from a typical residential septic system. Soil samples and ground water samples are analyzed for grain size distribution, composition and nutrients. My goal is to determine how the soil and plume interact along the flow path and how the nutrients change with respect to this interaction.

Denitrification in Long Island ground water:

Denitrification means the gaseous loss of nitrogen by either biological or chemical mechanisms. It prefers certain environments, including high organic matter in the soils, low oxygen concentration, neutral or even alkaline pH and certain temperature. Researches of Long Island ground water suggested the soil of Long Island are well aerated and have dissolved oxygen in ground water. So it has been believed for a long time that there isn't denitrification in Long Island ground water.

Eh represents the oxidizing character of the ground water. The lower the Eh value, the higher potential there is for denitrification. Previous research suggests that the calculated Eh based on NO3-/NH4+, NO3-/NO2- and NO2-/NH4+ redox pairs generally gives a higher Eh as compared to field measurements on the same sample (Fig.2).


Figure 2. Comparison of field-measured groundwater Eh value and potentials computed from the concentration of individual redox couples. (Langmuir, 1997)

I developed the Eh-pH diagram for dissolved oxygen equilibrated with water and NO3-/NH4+, NO3-/NO2- as well as NO2-/NH4+ . Figure 3 shows that there is a potential for denitrification in Long Island ground water. Nitrogen gas is stable within the NO3-/NO2- field. 

 

 

 

 

 

 

 

 

 


Figure 3. Eh-pH diagram for ground water data of Long Island from Suffolk County Water Authority.

Dissolve gas in ground water:

Denitrification, as a major process to decrease nitrate concentration in the nature, has been widely investigated all over the world. Several ways of tracing denitrification are used: (1) concentration change of N species, (2) isotopes, (3) dissolved “excess” N2 gas and other gas concentrations and (4) a mass balance approach. These methods are used separately or combined. With the development of Membrane Inlet Mass Spectrometry (MIMS), higher resolution makes the determination of dissolved gas concentration in waters possible. MIMS has been used to determine nitrous oxide, methane and precise measurement of gas ratio to determine denitrification rates both in fields and incubation experiments.

Sources of nitrogen gas in ground water are atmosphere and biologically generated by microbial denitrification. Ar concentration in ground water is from atmospherically derived Ar and radiogenic Ar. The ratio of N2/Ar in ground water in equilibrium with air is controlled primarily by temperature and pressure. However, since pressure change has equivalent results on dissolved Ar and N2 while temperature variation will cause small change on N2/Ar. Therefore, N2/Ar higher that presumed at air saturated water is considered to be resulted from excess N2 generated from denitrification or entrapped air gas bubble. With carefully designed delicate sample equipments, sampling procedure and high resolution measurement equipments, gas bubble can be eliminated. Thus, small change of N2/Ar measured is only coming from denitrification.

My object is to develop an improved ground water sample procedure to determine dissolved N2/Ar, and apply this method widely in Long Island ground water to investigate whether there is denitrification in Long Island ground water and the what the various degrees of denitrification are.

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Last updated: April 12, 2006.