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

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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.

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Fig 1 The location of the sampling sites along the northern bank of Waquoit Bay

Table 1 Groundwater characteristics of the sites Site Temperature Salinity DO DO% pH °C psu mg/L % saturated Upper Beach 10.8 0.1 6.9 63% 6.9 Lower Beach 10.5 0.1 5.9 53% 6.9 Before Barrier 11.5 0.1 6.9 65% 5.4 After Barrier 10.7 0.3 0.7 5% 6.7 Before Marsh 12.2 0.2 5.4 50% 6.5 Marsh 13.6 27.8 0.6 6% 6.7 Marsh Beach 11.3 24.6 1.1 11% 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 Before

Barrier

After Barrier

Concentration (µM) Concentration Upper Marsh Edge

Marsh

Marsh Beach

Time (hours)

Fig 6 Concentrations of bromide and nitrate during the push-pull experiments

Upper

Beach Lower Beach

After Barrier

Before

concentration (µM) concentration

Barrier

- 3

Upper Marsh

Edge Marsh Predicted Undiluted NO Undiluted Predicted

Marsh Beach

Time (hours)

Fig 7 Nitrate Concentrations corrected for dilution