My current project is focused on a septic tank plume from a
residential house in southern
. 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
. Acid rain, fertilizer usage on lawn, agriculture and pollution from sewage are
major concerns about chemical quality of
ground water. Increased nitrate concentration in
’s ground water has potential health hazard to infants and may result in
chronic toxicity and possible development of cancer.
has the highest concentration of cesspools in
. 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
is essential in water-management decision.
Septic Tank System:
Figure 1 is a typical septic tank system
. 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 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
ground water suggested the soil of
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
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
ground water. Nitrogen gas is stable within the NO3-/NO2-
Figure 3. Eh-pH diagram for ground water data of
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
ground water to investigate whether there is denitrification in
ground water and the what the various degrees of denitrification are.