Harmful of Brooklyn and Queens, respectively) in the west,

Harmful algal blooms (HABs) are a major environmental problem in
all 50 states. HABs are defined as the excessive growth
of various species of phytoplankton, including protists, cyanobacteria, and
macro and benthic algae whose proliferation negatively impacts water quality,
aquatic ecosystem stability, animal and human health. They can produce toxins and create conditions that kill fish and
other animals. The ecological stress from HAB’s can also create areas in water
with little or no oxygen where aquatic life cannot survive, called dead zones. The
tourism industry loses about $1 billion each year due to HAB’s, mostly through
losses in recreational fishing and boating activities. Moreover, commercial
fisheries lose 10’s of millions of dollars due to fish kills and contaminated
shell fish. Nitrogen availability is believed to be one of the leading causes
of the proliferation of HAB’s in coastal marine environments. In Long Island,
New York, nitrate levels in the Upper Glacial and Magothy Aquifers (ground
water) have increased by 40% and 200% respectively since 1987. Roughly 90% of
fresh water entering the coast is from ground water. This paper will
investigate the hydrology in Nassau and Suffolk counties and the nitrogen flux
throughout the watershed.

Description of
Long Island

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Long Island is a densely populated island off the East Coast of the
United States, beginning at New York Harbor just 0.35 miles (0.56 km) from
Manhattan Island and extending eastward into the Atlantic Ocean. The island
comprises four counties in the state of New York: Kings and Queens Counties
(which comprise the New York City boroughs of Brooklyn and Queens,
respectively) in the west, and Nassau and Suffolk counties in the east. This
paper will focus on the eastern side of Long Island (Nassau/Suffolk). The
completion of the Long Island Rail Road to Greenport in 1844 enabled the island
to become a major market-gardening center whose produce could be shipped to New
York City. Fishing, whaling, and oystering also remained important, but during
the second half of the 19th century the island became an attractive
recreational area for New York’s wealthy elite. Great estates and mansions were
built along the northern shore, and hotels that attracted thousands of summer
vacationers were constructed along the southern shore eastward from New York
City. Nassau and Suffolk Counties with close to 3 million people were and still
are completely dependent on groundwater for all of their freshwater needs.

Hydrology of
Long Island

The topography of Long Island is related to the last ice age, which
ended roughly 10,000 years ago (Franke 1972). The bedrock deposits of Long
Island are end products of the advance and melting of several ice sheets during
the Pleistocene Epoch. The lowermost
formation of Pleistocene age on Long Island is the Jameco Gravel, a
coarse-grained outwash deposit. Above the Jameco is the Gardiners Clay, a fossiliferous
marine interglacial formation composed mostly of beds of silt and clay (Buxton
1992). The beds above the Gardiners Clay consist of several sequences of outwash
and till. The unconsolidated materials that overlie the bedrock constitute Long
Island’s groundwater reservoir. Three major aquifers can be identified: the
Upper Glacial aquifer at the top, the Magothy aquifer in the middle and a deep
less accessible Lloyd aquifer lying just above the Paleozoic metamorphic
basement rocks.

There are two major confining units. The Pleistocene Gardiners Clay is
found mainly on the southern part of the island and provides some restriction
of flow between the Upper Glacial and the Magothy aquifers. The other confining
unit is the Raritan confining unit which is quite thick and restricts the flow
between the Lloyd and the Magothy aquifers. The flow of water is dominantly to the north or to the south of the
ground water divide along the center of the island west of William Floyd
Highway (Buxton 1992). Therefore, there
is little east-west mixing of the groundwater. On the east side of William
Floyd Highway there is significant flow eastward associated with the Peconic
River. The water moves laterally in the Upper Glacial aquifer to streams and
shoreline or moves downward through the Upper Glacial aquifer to the lower
units. Some of the water from the Magothy circulates downward and then flows
upward toward the shoreline and then into the Long Island Sound or Atlantic
Ocean (Böttcher et al., 1990). The rest mixes at depth with salt water under
the Long Island Sound and Atlantic Ocean. A very small percentage of the water
penetrates the Raritan confining unit and enters the Lloyd aquifer.

At the top of the Magothy, the water is about 10 years old. Near the
center of the Magothy it is 100 years old. Near the base of the Magothy the
water is about 500 years old. The Magothy is the source of much of Long
Island’s drinking water. Within the Lloyd aquifer the water is much more
ancient. Near the top the water is 1000 years old and as the
freshwater-saltwater interface it is approached beneath the Atlantic Ocean the
water is some 8000 years old (Buxton 1992). The ages of water help to conceptualize
the amount of time it would take to naturally flush out any pollutants.

Sources of
Nitrogen to Groundwater

Typically, the amount of nitrate in groundwater is
related to land use, where the greatest concentrations are observed in
agricultural regions. Nitrogen percolates easily into the groundwater through
the soil along with rainwater recharge or irrigation water. As a result, the
shallow aquifers are more likely than deeper ones to initially suffer from
contamination problems. Previous research has demonstrated that ?15N
nitrate and isotopic composition of groundwater nitrates can be used to help
distinguish among different nitrate sources (Kendall
et al., 1997). In addition, the oxygen isotopes can be used to identify
processes, such as denitrification, that may alter the concentration and
isotopic composition of nitrate (Amberger and
Schmidt, 1987). It was shown that the main sources of nitrate in groundwater in
developed areas of Suffolk County are turfgrass fertilizers and wastewater via
septic tank/cesspool systems and discharge from sewage treatment plants (Flipse
et al., 1984; Kimmel, 1984).

Farming was extensive on Long Island before World
War II but since then development has spread eastward from New York City, and a
high proportion of the land is now used for residential purposes. In 1981
turfgrass occupied 25% of Suffolk County (Koppelman et al., 1984), either as
golf courses, parks and residential or commercial lawns. Suffolk County Water
Authority estimates 21 million gallons/day, or 30% of the water pumped is used
for the sole purpose of lawn irrigation (Wayland, K.G, 2003). Nitrogen is a
major nutrient needed to keep turfgrass healthy and green but has a consequence
of groundwater pollution.

Table 1: Groundwater recharge in Long Island

Infiltration
Source

Million
Gallons per day

Precipitation

1,130

Septic
tank/cesspools

84

Sewage
treatment plants

24

Water
used for irrigation

21

Most of Suffolk County is not sewered. Instead most
homes have septic tank systems that discharge their waste water back to the
groundwater system. As a result a relatively small percentage of the groundwater
recharge in Suffolk County is lost, about 10%, compared to 55% for Nassau
County. The most serious problem when using septic tanks is the introduction of
nitrates into the ground water. When sewage is discharged to a septic tank or
cesspool, some nitrogen is lost as ammonia or nitrogen gases and about half is
oxidized to nitrate. On Long Island, ?18O for nitrates produced by
nitrification of ammonium would be expected to range from approximately +2.5 to
+ 3.2‰ assuming an isotopic composition of +23.5‰ for atmospheric oxygen (Amberger
and Schmidt, 1987) and -7 to -8‰ for Long Island groundwater U. S. Geological
Survey Waterstore data. The ?18O nitrate values in the public
supply wells indicate that most nitrate is derived from nitrification of
ammonium. Moreover, the ?15N isotope signature of the septic
affected waters were distinguishably heavier than the fertilizer ?15N  signature. In a recent interview, Dr. Chris
Gobler, a professor at Stony Brook University’s School of Marine and
Atmospheric Sciences and a nitrogen pollution expert says, “High levels of
nitrogen – associated with residential septic tanks and cesspools and
fertilizer runoff from agricultural lands – in the groundwater has led to the
degradation of local drinking water supplies as well as Long Island’s coastal
ecosystems”(Rabin 2012).

Effect of Nitrogen
in Ground Water

Six percent of the wells in Nassau exceed 10 mg per
liter of nitrate that is the EPA maximum allowed for drinking water. Such wells
are either abandoned or the water from the well is blended with that from a
well that has a lower nitrate content. Drinking water with elevated nitrate
levels is detrimental to human health and is associated with respiratory and
reproductive system illness, some cancers, thyroid problems and even “blue
baby syndrome.” From an ecological standpoint, too much nitrogen and
nitrate runoff can cause eutrophication, or nutrient loading in surface and
marine waters that result in algal blooms that create those notorious
oxygen-starved “dead zones” and “red tides” that kill off
aquatic life. Long Island has experienced annual harmful algae bloom since the
spring of 2004. These events result in a huge loss in revenue for fisheries and
coastal real estate due to un-pleasurable conditions.  Among other things, excess nitrogen
contributes to two notable problems in coastal Long Island waters: the
proliferation of macroalgae (specifically Ulva, or “sea lettuce) and extensive
damage to the marsh grasses and their sub structures that, in turn, are
integral to maintaining natural shoreline protection against coastal storm
surge and waves. 

Mitigation
Strategies

Some people are concerned about drinking water quality and others about
waste and surface water. Nonetheless, more emphasis should be placed on
developing preventative measures to water pollution instead of remediation
efforts. Groundwater contamination with nitrate is a prime example of the
difficulties in addressing nonpoint source pollution (where there is no single
source of attributable pollution but many contributors). With numerous sources
covering a widespread area, nonpoint source pollution makes it difficult to
track. Many ideas are being put forward to solve long islands nitrate water problems.
I tend to favor the plans that include management of sources, but some
remediation techniques can be useful.

Treatment of ground water for nitrates can be done inside the ground
(in-situ) or outside the ground (ex-situ). Pump-and-treat, a type of ex situ
remediation, refers to the extraction of contaminated water from the subsurface
followed by treatment with denitrifying bacteria and subsequent discharge of
treated water to groundwater or surface water (King 2012). Water that is
pumped from the subsurface comes from the highly conductive materials,
while water within areas of low conductivity is removed much more slowly. Thus,
reinjected, clean water will mix with untreated water and diffusion of nitrate
from more concentrated to the less concentrated waters will prolong remediation
efforts. This technique is a lengthy process, with high cost (construction and
energy) and diminishing returns (King 2012). The same dilution factor would
result with less expense if nitrate sources were reduced. A similar technique
is used by drinking water municipalities in Long Island. To reduce nitrate
levels, connections are created such that the contaminated well water can mix
with cleaner well waters, thereby reducing nitrate levels by dilution (source). These techniques can reduce the symptoms of
nitrate loading but do not address the source.

An emerging technique in the field of groundwater nitrate removal is the
permeable reactive barrier (PRB). PRBs can be used inside the groundwater layer
to remove nitrate from groundwater through biological denitrification or
chemical denitrification. Denitrification, a process by which nitrate is reduced
to nitrogen gas, is one of the only ways to remove nitrate from water. This
process can be facilitated by bacteria or by metals such as zero valent iron
(ZVI). Nano particles of ZVI are coated onto sand and then used on PRBs to
reduce nitrate as it interacts with its surface (King 2012). There is no energy
cost to operate it because it works with the flow of groundwater. This is a
useful method for nitrate sequestration because it can last for up to 10 years.
The biggest limitation of this technique is expense at plume depths greater than
30 ft. I suggest that this method be expanded for use in domestic wastewater
treatment systems. The reactive barrier can convert and remove nitrate from
septic effluent before it can contaminate surrounding water bodies.

Given the fact the septic systems are identified as a source of nitrate pollution,
a significant effort should be made towards the development of septic systems
capable of denitrification. The Long Island Nitrogen Action Plan recommends the
development of denitrifying septic systems but there has not yet been a model
agreed upon. Since the contaminant of interest is nitrate, an ideal system
should include a drain field capable of supporting two conditions, aerobic and
anaerobic.

The septic effluent is released with ammonium and must undergo
nitrification and then denitrification to be completely removed of dissolved
inorganic nitrogen. This can be achieved by increasing water retention time in
the soil matrix, unlike the current drain fields that are built for rapid drainage.
The cycling of nitrogen in soil is driven by microbial metabolism and plant
uptake processes. A prolonged interaction between the effluent and the soil
will enhance the amount of nitrogen uptake. The dissolved carbon in the septic effluent
will react with oxygen and create an anaerobic layer during retention that is conducive
for denitrification. Moreover, the PRB technology can be integrated into the
soil matrix of septic drain fields in order to maximize nitrogen removal. A mix
of strategies will need to be applied to accomplish the overall goal of
nitrogen management. Septic drain fields are the dumping ground for dissolved
nitrogen. The system would benefit from maintaining the appropriate soil
content, vegetation, water level and retention time necessary for optimum
nitrate removal. 

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