Download The Ability of Natural vs. Artificial Riparian Zones to Filter nutrients

The Ability of Natural vs. Artificial Riparian Zones to Filter nutrients assessed by
push-pull experiments
Timothy Sinkovits
Ripon College
December 2011
Abstract
Anthropogenic nitrogen loading is causing widespread eutrophication in estuaries across
the United States. While salt marshes reduce the amount of nitrogen in the groundwater, urban
expansion has caused the loss of most of the salt marshes. A NITREX permeable reactive
barrier is being tested for its ability to remove nitrate from groundwater at Waquoit Bay, MA. A
salt marsh, a beach with the barrier and a control beach were analyzed using push-pull
experiments, soil cores, and groundwater analysis. The push-pull experiments found
denitrification only happening in the upper edge of the salt marsh. But evidence of
denitrification was found to occur in both the salt marsh and the barrier with barrier filtering
more of the total nitrogen than the salt marsh.
Introduction
Groundwater seepage between terrestrial and aquatic systems is a major source of
nutrient loading to aquatic ecosystems via groundwater seepage. However, riparian zones have
been shown to filter nutrients in the groundwater, thereby reducing the overall nutrient load to
the aquatic system (Lowrance et al. 1984). Most importantly riparian zones filter nitrate from
the groundwater which is a nutrient that in excessive amounts causes eutrophication in marine
coastal zones (Simmons et al. 1992, Addy et al. 2002). Unfortunately, humans have increased
the nitrate loading to marine systems by waste water, fertilizer, and land clearing, and have
caused many estuaries to show symptoms of eutrophication. In fact, 78% of all estuary area is
categorized as showing moderate to severe symptoms of eutrophication (Bricker et al. 2008).
Due to the increased risk of eutrophication, riparian zones have become invaluable in preserving
our estuaries.
One of the most common riparian zones along the coastal United States is the salt marsh.
While being able to filter pollutants from the ground water, salt marshes have many other
benefits such as being a nursery for fish and invertebrates and preventing soil erosion (Valiela et
al. 2009). Unfortunately, urban development and global warming have lead to large losses of
salt marshes along coastal regions. Estimates of coastal wetland loss have varied from 13-31%
globally, and more recent statistics released in 2007 seem to indicate a 30% loss in wetlands
globally. New England alone lost 50% of the salt marsh area by the mid-1970s (Valiela et al.
2009). While wetland restoration is becoming popular, studies have concluded that
reconstructed marshes are less effective at filtering nutrients than natural salt marshes
(Thompson et al. 1995).
However, estuaries in regions where salt marshes have disappeared could still be
protected from eutrophication by installing an artificial riparian zone. An example of an
artificial riparian zone can be the NITREX™ permeable reactive barrier (PRB) which was
developed by Dr. Will Robertson at Waterloo University. A 20 by 3.5 by 2 meter NITREX™
barrier composed of a mixure of woodchips and lime was installed at the Waquoit Bay National
Estuarine Research Reserve in August 2005 under the guidance of Dr. Joseph Vallino and Dr.
Kenneth Forman (Vallino and Foreman 2008). The barrier functions by driving the ground
water anoxic by the decomposition of the woodchips. This decrease in oxygen forces microbes
within the barrier to use nitrate as an electron acceptor instead of oxygen. During this process
the nitrate is reduced into nitrogen gas (N2), a process known as denitrification.
The NITREX™ barrier technology, however, has not been tested on removing nitrate
from groundwater before on a large scale, and thus NITREX barriers have never been compared
to the natural nitrogen filtering properties of a healthy salt marsh. In this study, I will compare
nitrate removal in groundwater passing across a natural salt marsh, an unaltered control beach
and a beach altered by an installed NITREX PRB in the riparian zone.
Methods
I employed the push-pull method described by Istok et al. (1997) with a few
modifications at sites along Waquoit Bay. I sampled water from seven locations. These
included experiments up and down gradient at three sites: a control beach, a beach with a
NITREX barrier installed, and a naturally occurring Spartina marsh. I also performed an
experiment within the salt marsh.
At each location, I inserted a minipiezometers approximately 1 m into the soil at the
seven sites (Fig 1). I used a peristaltic pump and a Hydrolab Quanta to extract and measure the
temperature, salinity, pH, dissolved oxygen, and percent oxygen saturation of the groundwater.
Then I drew up 10 L of water into a 20 liter carboy. This water was enriched with 10 mL of a
0.5005 M NO3- (as KNO3) and 0.5005 M Br- (as KBr) solution to achieve a final concentration
elevated 500 µmolar over ambient and then pumped back into the ground.
I collected 120 mL of samples of groundwater before adding the nutrients to the
groundwater and 60 mL of samples after adding the nutrients. All samples were filtered through
glass fiber filters (GFF) and stored on ice in the field. I then collected a “Time 0” sample of
60mL five minutes after pumping the enriched groundwater solution and approximately every
hour thereafter for 6 to 7 hours.
I analyzed the water samples for ammonium, nitrate, phosphate, and bromide using
colorimetric methods. A modification of the method of Wood et al. (1967) was used to measure
the nitrate concentration on a Lachat flow injection analyzer. The ammonium concentration was
measured using a modification of the method by Solarzano (1969). I used a modified method by
Murphy and Riley (1962) to measure the phosphate concentration and the bromide concentration
was measured by a modified method from Presley (1971).
In addition to water samples, I extracted soil cores a few days before the push-pull
experiments from roughly the same location as each minipiezometer. I analyzed the soil cores
for carbon and nitrogen stocks, wet-dry ratio, and nitrogen mineralization. I measured the
available nitrate and ammonium from the soil cores using the KCl extraction technique. The soil
cores were incubated for 14 days in a temperature-controlled growth chamber set to 25 °C. At
the end of the incubation period, I measured the available nitrate and ammonium again and used
the difference from initial and final to calculate net mineralization. I also ground dried soil
samples and analyzed them through a CHN analyzer to determine the C and N stocks. Using the
net mineralization and the total nitrogen lost from the groundwater, I calculated the net nitrogen
removed and the net nitrate removed.
Push-Pull Dilution Calculations
I subtracted the groundwater concentration of bromide and nitrate from the measured
bromide and nitrate during the push-pull experiments to yield the added concentration of
bromide and nitrate. Then for each time period, I found a constant such that the constant times
the bromide concentration at that time point would yield the bromide concentration at time 0. I
then multiplied the nitrate concentration by that constant to yield the nitrate concentration as
expected if it was not diluted.
Results
Groundwater Characteristics
All of the sites were freshwater (<1 psu) except for the marsh site and the marsh beach
site (Table 1). Also, the groundwater was oxic except for the salt marsh, the marsh beach, and
below the barrier sites which were hypoxic. The temperature was roughly the same between
sites with the range being between 10 and 14 °C.
The ambient groundwater bromide concentrations were almost 600 times greater in the
marsh and marsh beach than the other sites (Fig 2). Bromide concentrations were highest in high
salinity sites.
No major change in the nitrate concentrations were observed between the upper and
lower control beach sites. However, nitrate completely disappeared in the groundwater after the
barrier. Similarly, groundwater in the salt marsh and the marsh beach had a lower nitrate
concentration than the groundwater entering the marsh.
I found the ammonium concentration within the marsh to be 10 times greater than the
marsh beach concentration and 80 times greater than the other sites. I also noticed an increase in
phosphate concentrations in the sites with hypoxic groundwater.
Soil Core Analysis
The soils contain very low percent carbon values for all of the sites except for within the
marsh (Fig 3). Soils deeper below the surface of the marsh were measured to have lower carbon
content than the surface. There is a similar trend with the nitrogen content of the soil.
I observed a lower mass of extractable NO3--N in the soil in the lower beach soil than the
upper beach soil (Fig 4). A similar trend is seen in the soil from the barrier site especially the
soil at 45cm depth after the barrier which contained no observable nitrate and ammonium
nitrogen mass. There is no noticeable trend in the nitrate concentrations between the soils in the
salt marsh. All sites have low NH4+-N mass
I found low amounts of N mineralization in all of the cores for the soil taken from the salt
marsh (Fig 5). The cores from depths 15cm and 100cm had a net decrease in N by 1265 and 106
µg N kg-1 dry soil day-1 and the core from the 45cm depth has a net N mineralization of 162 µg
N kg-1 dry soil day-1.
Push-Pull Experiments
For the control beach and the barrier sites, both bromide and nitrate concentrations
decreased rapidly and disappear after only a few hours (Fig 6). The bromide concentrations for
the upper marsh edge site were fairly constant but the nitrate concentrations decreased over time.
I did not observe a decrease in either the bromide or the nitrate concentrations for the site within
the salt marsh. The experiment on the marsh beach yielded trends similar to the other sites on
the beach.
I did not notice a trend in either the ammonium or phosphate concentrations over the
course of the experiments for all of the sites.
After being corrected for dilution, only the upper salt marsh edge site showed any
decrease in nitrate over time (Fig 7). The denitrification rate for this site is 194 µg N kg-1 dry
soil hr-1.
Discussion
The Control Beach
Both the ambient groundwater concentrations and the push-pull experiments show no
evidence of nitrogen loss between the upper and lower beach. Only the nitrogen extracted from
the soil cores show a decrease in nitrate between the upper and lower beach. However, because
a majority of the data suggested that nitrogen reduction did not occur on the control beach, I
conclude that beaches without a riparian zone do not filter nitrogen. This lack of nitrogen
reduction is likely because the lack of organic matter in the soil and presents of oxygen in the
groundwater is not a suitable environment for denitrifying bacteria.
The NITREX Barrier
The decrease in exractable nitrate from the soil and within the groundwater from before
to after the barrier suggests that denitrification occurs. However, lack of denitrification over the
course of the push-pull experiments implies that denitrification does not happen below the
barrier. Thus denitrification is present within the limits of the barrier. While the groundwater
below the barrier is hypoxic, the lack of organic matter is probably not enough to sustain a
sufficient population of denitrifying bacteria. This conclusion appears to contradict the findings
of Collin Knauss (2011), but I only sampled a meter below the surface where Knauss measured
below a meter from the surface. Future experiments need to have push-pull experiments at lower
depths to confirm my speculated results.
The Salt Marsh
I did not see any evidence for denitrification from the extractable nitrate in the soil, but
the lower nitrate concentrations from the groundwater within the marsh and the marsh beach
suggest that denitification is occurring. Also, the losses in nitrogen during the incubations
suggest that immobilization or denitrification is occurring in the salt marsh sediments. However,
the results from the push-pull experiments did not show any net denitrification within the marsh
or the marsh beach despite both environments having hypoxic groundwater. The push-pull
experiments show that net denitrification is only occurring in the upper salt marsh edge site.
Thus net denitrification probably happens along the up gradient edge of the salt marsh.
The low concentrations of dissolved oxygen and the high percent of organic matter in the
soil of the salt marsh should be enough to sustain a population of denitrifiers, but the high
concentrations of ammonium suggest that the denitrifyers could have been outcompeted by
anaerobic ammonium oxidizing (anammox) bacteria. Anammox bacteria convert ammonium
and nitrite into nitrogen gas and water. While competition between anammox and denitrifying
bacteria has been reported, the high growth rate of denitrifiers should outcompete the slower
growing anammox bacteria (Chamchoi et al. 2007). Also, anammox has not been discovered to
contribute a significant amount dinitrogenous gas emitted from the sediments of a salt marsh
(Koop-Jakobsen and Giblin 2009). Future investigations into the interactions between anammox
bacteria and denitifyers within the salt marsh are needed to clarify these issues.
The marsh beach likely does not exhibit any denitrification because the marsh beach is
lacking in organic matter similar to the barrier and control beach sites.
Barrier and Salt Marsh Comparison
From the groundwater, the barrier reduces the total amount nitrogen flowing through it by
82%, but the salt marsh only reduces the total amount of nitrogen by 44%. Also, whereas the
barrier removes 100% of the nitrate, the salt marsh only removes 70% of the nitrate.
Issues with the Experiment
The push-pull experiments performed yielded unconclusive results. The reason for the
rapid loss of the tracer within the sites on the beach can be explained by the tide. An experiment
completed in the summer of 2007 by Richard McHorney shows that the tide affects the depth of
the groundwater. From high to low tide the groundwater was found to fall 30 cm lower. The
minipiezometer only had a range of about 10cm so the nutrient plume could have easily move
out of sampling range.
Future Projects
Because the tide proved to affect the height and the flow of the groundwater, a few
adjustments to this experiment is needed for increased accuracy. The push-pull method can be
used along a freshwater with increased confidence because of the lack to tides.
If samples must be taken along tidal water, microcosms should be used instead of the
push-pull experiments as this would negate the effects of the tide. However, several replicates of
the microcosms should be used because patchiness in groundwater denitrification has been
observed (Gold et al. 1998). If a push-pull experiment must be used, I suggest using multiple
sampling points around the injection point to catch the nutrient plume if it were to drift away
from the injection point.
The salt marsh also the population of anammox and denitrifying bacteria needs to be
quantified at each site to confirm my suspicions about anammox bacteria outcompeting the
denitrifying bacteria.
Conclusions
The control beach does filter nitrogen, but the salt marsh and the NITREX PRB both
displayed evidence of denitrification. While the groundwater concentrations seem to suggest
that the barrier filters a greater amount of nitrate and the total nitrogen in the groundwater than
the salt marsh. However, without an actual rate of nitrate reduction and residence time of the
groundwater within the riparian zones I cannot fully conclude if the barrier filters at least as
much nitrogen as the salt marsh.
References
Bricker, SB, Longstaff, B, Dennison, W, Jones, A, Boicourt, K, Wicks, C, and J Woerner. 2008.
Effects of nutrient enrichment in the nation’s estuaries: A decade of change. Harmful
Algae 8: 21-32.
Chamchoi, N, Nitisoravut, S, and JE Schmidt. 2007. Inactivation of ANAMMOX communities
under concurrent operation of anaerobic ammonium oxidation (ANAMMOX) and
denitrification. Bioresource Technology 99: 3331-3336.
Gold, A, Jacinthe, P, Groffman, P, Wright, W, and R Puffer. 1998. Patchiness in groundwater
nitrate removal in a riparian forest. J. Environ. Qual. 27:146-155.
Istok, J, Humphrey, M, Schroth, M, Hyman, M, and K O’Reilly. 1992. Single-well, “push-pull”
test for in situ determination of microbial activities. Ground Water 35(4):619-631.
Knauss, C. 2011. Characterizing microbial denitrification nirS gene abundance in relation to
biogeochemical processes and characteristics up-gradient, within and down-gradient from
a NITREXTM Permeable Reactive Barrier in Waquoit Bay, MA.
Koop-Jakobsen, K, and AE Giblin. 2009. Anammox in tidal marsh sediments: The role of
salinity, nitrogen loading, and marsh vegetation. Estuaries and Coasts 32: 238-245.
Lowrance, R, Todd, R, Fail, J, Hendrickson, O, Leonard, R, and L Asmussen. 1984. Riparian
forests as nutrient filters in agricultural watersheds. BioScience 34(6):374-377.
Murphy, J, and J Riley. 1962. A modified single solution method for the determination of
phosphate in natural waters and uses ascorbic acid instead of stannous chloride. Anal.
Chem. Acta 27:31-36.
Presley, BJ. (1971) Appendix: Techniques for analyzing interstitial water samples. Part I:
Determination of selected minor and major inorganic constituents. In Winterer et al.,
1971, Initial reports of the deep sea drilling project, volume III. Washington, US
Government Printing Office, pp. 1749-1755.
Thompson, SP, Paerl, HW, and MC Go. 1995. Seasonal patterns of nitrification and
denitrification in a natural and a restored salt marsh. Estuaries 18(2): 399-408.
Simmons, R, Gold, A, and P Groffman. 1992. Nitrate dynamics in riparian forests: groundwater
studies. J. Environ. Qual. 21:659-665.
Solarzano, L. 1969. Determination of ammonium in natural waters by phenol hypochlorite
method. Limnol. Oceanogr. 14:799-800.
Valiela, I, Kinney, E, Culbertson, J, Peacock, E, and S Smith. 2009. Chapter 4: Global Losses of
mangroves and salt marshes. Global Loss of Coastal Habitats: Rates, Causes and
Consequences. Fundación BBVA, Pg. 109-142. Carlos Duarte(ed.)
Vallino, J, and K Foreman. 2008. Effectiveness of reactive barriers for reducing N-loading to the
coastal zone.
Wood, E, Armstrong, F, and F Richards. 1967. Determination of nitrate in seawater by cadiumcopper reduction to nitrite. J. Mar. Biol Assoc. U.K. 47:23
Fig 1 The location of the sampling sites along the northern bank of Waquoit Bay
Table 1 Groundwater characteristics of the sites
Site
Upper Beach
Lower Beach
Before Barrier
After Barrier
Before Marsh
Marsh
Marsh Beach
Temperature Salinity
°C
psu
10.8
0.1
10.5
0.1
11.5
0.1
10.7
0.3
12.2
0.2
13.6
27.8
11.3
24.6
DO
mg/L
6.9
5.9
6.9
0.7
5.4
0.6
1.1
DO%
% saturated
63%
53%
65%
5%
50%
6%
11%
pH
6.9
6.9
5.4
6.7
6.5
6.7
6.5
Fig 2 Groundwater nutrient concentrations in µM for each site
Fig 3 Carbon and Nitrogen percent of the soils
Fig 4 Extractable NO3--N and NH4+-N from the soils in µg N/kg dry soil
Fig 5 Net N mineralization after a 2 week incubation at 25°C
Upper
Beach
Lower
Beach
Bromide
Nitrate
After
Barrier
Concentration (µM)
Before
Barrier
Upper
Marsh
Edge
Marsh
Marsh
Beach
Time (hours)
Fig 6 Concentrations of bromide and nitrate during the push-pull experiments
Predicted Undiluted NO3- concentration (µM)
Upper
Beach
Lower
Beach
After
Barrier
Before
Barrier
Upper Marsh
Edge
Marsh
Marsh
Beach
Time (hours)
Fig 7 Nitrate Concentrations corrected for dilution