CICEET Progress Report for the period 3/16/06 Through 9/15/06

Project Title: Effectiveness of Reactive Barriers for Reducing N-Loading to the Coastal Zone
Principal Investigator(s): Joseph Vallino and Kenneth Foreman
Additional Investigator(s): Pio Lombardo
Project Start Date: 1 Sept 04

Figures

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Tables

Table 1

Tasks to meet objectives

• Sample wells in PRBs at both sites.
• Analyze well samples for constituents.
• Use membrane inlet mass spectrometer (MIMS) to measure N₂/Ar ratios of samples.
• Setup microcosms for continuous and periodic saltwater additions.
• Construct stoichiometric balance around microcosm PRBs to assess dominate metabolic processes.

Progress on Tasks

• In mid May 2006, both PRB sites were sampled for dissolved nitrogen constituents as well as DO, salinity, and temperature.
• Many well samples have been analyzed (see data below), but some sample analyses remain to be completed.
• The MIMS instrument has been sent back to the manufacture for repairs, so we have been unable to measure N₂/Ar ratios.
• Microcosms were re-setup and the saltwater experiment began in mid June and ran for approximately 6 weeks.
• Microcosms samples were analyzed and stoichiometric balance calculations were completed.

Have the results/data gathered during this reporting period changed the project objectives when compared to your original proposal?
Our results from the Apicella PRB installation (see below) show that part of the nitrate groundwater plume is passing under the PRB. This indicates that we need to develop novel and economically viable ways to install the PRB to deeper depths. For the current project, we used standard trench box technology that limited the installation depth to about 2 m below the water table in the medium to coarse sandy environments on Cape Cod. These results have not changed our objectives, but they do indicate areas for technology transfer research.

Dissemination activities during this reporting period

• Project related presentations/poster sessions at workshops/conference An NSF Research Education for Undergraduates (REU) student presented work from his microcosms research at the MBL General Science Meeting in Aug, 2006.
• Contact with End Users We indicated in our proposal that the town of Falmouth is the end user for this technology. Falmouth has 19 coastal bays and estuaries that are being impacted by nutrient and wastewater inputs and faces water quality and nutrient management challenges typical of many coastal towns in New England and the Mid-Atlantic States. It is, therefore, a good model to test how the barrier approach can be incorporated into municipal wastewater treatment plans.
  Ken Foreman is a member of the town of Falmouth planning board and the planning board’s representative to the municipal Nutrient Management Working Group (NMWG). Foreman has been in routine contact with the Falmouth Town Planner, Brian Currie, the town administrator, Robert Whritenour, and the town wastewater superintendent, Amy Lowell. Foreman met with the NMWG several times in July and August to discuss the town’s comprehensive wastewater management strategy and if we can demonstrate their effectiveness, stability and cost-benefit, we envision barriers being incorporated into this strategy in some way (see Appendix I).
• Student activity (e.g. theses, dissertations, etc.) on the project (please identify students as graduate or undergraduate) During this period, one undergraduate student (Mark Anderson) from San Francisco State University conducted in NSF REU sponsored research project investigating the impact of continuous and periodic saltwater intrusion on the functioning of the NITREX PRB medium using laboratory microcosms.

Difficulties
Our membrane inlet mass spectrometer (MIMS) has been malfunctioning and had to be sent back to the manufacturer for repair. Consequently, we have been unable to measure N₂/Ar ratios. These measurements are critical to demonstrate conclusively that nitrate removal by the NITREX™ PRBs is due to denitrification and not nitrate immobilization on the NITREX medium itself by either biotic (Moorhead et al. 1999) or abiotic processes (Davidson et al. 2003). Although we have numerous samples stored for N₂/Ar measurement from both field-based PRBs and the laboratory microcosm PRBs, at this time it is not known how long the samples can be stored before they are compromised due to gas leakage. As a result, it may be necessary to resample the PRB wells to obtain fresh samples once the MIMS machine is back on-line. Because of the MIMS problems, we have requested a one-year no cost extension.

Data Generated to date
Microcosm Experiments
Although the samples from the Waquoit Bay PRB are still being analyzed and data post processed, previous salinity data collected at this site indicated periodic, tidally induced, saltwater intrusion into the PRB was occurring (Figure 1). To investigate the impact of periodic saltwater intrusion on the functioning of the NITREX™ PRB medium, we employed our previously constructed microcosms that consist of columns of NITREX™ medium that can be perfused with water augmented with nutrients as desired (Figure 2). The three treatments investigated consisted of 1) groundwater collected from a 2.5 m deep well at Waquoit Bay and augmented with 250 µM NO₃^-, 2) Woods Hole saltwater augmented with 250 µM NO₃^-, and 3) a periodically changing flow of 8 hours of nitrate augmented groundwater followed by 4 hours of nitrate augmented seawater. Each treatment was run in duplicate.

Microcosm results clearly show that nitrate concentration was reduced to 1.0 mM or less in each treatment, so the presence of seawater does not appear to affect NITREX™ nitrate removal (Figure 3 and Figure 4). However, treatments receiving saltwater, or periodically receiving saltwater, did produce small amounts of ammonium at approximately 5% of total inorganic nitrogen input (Figure 5). Since we observe some hydrogen sulfide production in the seawater treatments (Figure 6), it is likely that some dissimilatory nitrate reduction to ammonium (DNRA) is occurring as a result of seawater sulfate reduction (Brunet and Garcia-Gil 1996).

The microcosms were also monitored for net dissolved inorganic carbon (DIC) production (Figure 7), so that we could assess the contribution of the five dominant respiratory reactions associated with NITREX™ decomposition (Table 1). From net production of ammonium (Figure 5) and hydrogen sulfide (Figure 6) and net removal of nitrate (Figure 3), we estimated the contribution of reactions 1-4 (Table 1) to total DIC production (Figure 8). Although significant DIC production occurs from reactions 1-4, it is clear that significant DIC production must be occurring from other reactions (Figure 8), the most likely being fermentation (Reaction 5, Table 1). However, it is possible that our estimate of DIC production associated with sulfate reduction may under estimate true sulfate reduction because some hydrogen sulfide may have reacted with other constituents in the microcosm, such as iron. The DIC balance also shows that the presence of seawater (either continually flowing or pulsed) increases the overall rate of NITREX™ oxidation as compared to the pure groundwater case (Figure 7 and Figure 8), which could reduce the useful life expectancy of the PRB by 20 to 30%.

Child’s River Field-Based PRB
The May 2006 samples collected from the Apicella PRB (located along the Child’s River) have been analyzed and the data post processed. A cross-section of nitrate concentration and temperature reveals that a near surface nitrate plume is being removed by the NITREX™ PRB; however, a deeper nitrate plume passes beneath the PRB largely unaltered (Figure 9). Temperature profiles indicate some upward flow into the PRB due to its higher hydraulic conductivity, but the vertical flow is insufficient to capture the deeper nitrate plume (Figure 9). As expected, dissolved oxygen (DO) both within and down gradient of the PRB is hypoxic or anoxic (Figure 10). Measurements of specific conductivity (Figure 11) indicate that the PRB may experience some saltwater intrusion, which could explain the elevated levels of ammonium observed both within and down gradient of the PRB (Figure 12). These results are consistent with our seawater infused microcosm experiment, but we need to confirm that the elevated specific conductivity is indeed caused by saltwater and not lime that is a component of the NITREX™ medium.

Preliminary Conclusions
Although our project has not formally completed, at this time we can draw the follow preliminary conclusions. There is no question that NITREX™ effectively removes nitrate from groundwater even when groundwater is contaminated with low concentrations of nitrate (~ 100 µM), such as occur in diffuse discharges along the coast. Furthermore, NITREX™ effectively removes nitrate even during saltwater intrusion events; however, it does appear that some nitrate is converted to ammonium (~5%) if hydrogen sulfide is present and seawater does increase decomposition rate of the NITREXTM media. The details of PRB hydrology and biogeochemistry on tidal time scales remain uncertain, as we lacked real time instrumentation needed to resolve these processes in the field.

At this time, we are unable to determine if nitrate removal occurs via denitrification or N immobilization on NITREX™, but this should be rectified once the MIMS instrument is repaired and we can analyze stored samples for N₂/Ar ratios. Although simple calculations indicate the NITREX™ PRB could last for several hundred years at current N loading, our two-year study is insufficient to determined effective life of the PRB under standard field conditions.

In regards to groundwater nitrate interception along the coast, our results illustrate the importance of PRB design and installation. Although groundwater nitrate plumes are shallow at the coastal boundary, it can be difficult installing a PRB deep enough to intercept the nitrate plume using conventional trench box excavation technology. This challenge is evident at the Child’s River (aka Apicella) PRB site, where the second deeper nitrate plume passes unaltered under the PRB (Figure 9). Different installation technologies that allow for deeper PRBs with minimal surface and environmental impact require further research.

To summarize, we believe the NITREX™ PRB could be effective at removing nitrate-contaminated groundwater prior to its discharge into coastal embayments provide the PRB intercepts the nitrate plume. We recommend further research in the following areas: 1) characterization of tidally induced circulation within and surrounding the PRB and its impact on PRB biogeochemistry; 2) environmental impact of low DO and elevated dissolved organic matter in water discharging from the PRB to coastal environments; 3) the relative importance of N immobilization versus denitrification over time; 4) different PRB installation technologies that minimize saltwater intrusion and intercept deeper nitrate plumes with minimal environmental footprint; 5) demonstration of NITREX™ PRB in bioremediation of eutrophied coastal embayments dominated by groundwater discharge; 6) estimation of effective lifetime of NITREX™ PRBs. Some of the above research objects could be conducted at the two currently installed NITREX™ PRB sites, while others would require pilot-scale installations around a small embayment.

Project Objectives for Next Reporting Period

Objectives
During the one-year no cost extension, we plan to analyze previously stored samples for their N₂/Ar ratios so we can determine the extent of N immobilization versus denitrification.

Work plan to Meet Objectives
The MIMS instrument needs to be repaired.

Dissemination Objectives for next reporting period
Falmouth along with all other Cape Cod towns and many other coastal communities is facing increasing pressure to mitigate nutrient inputs to coastal water bodies. The Massachusetts Estuaries Project and Massachusetts Department of Environmental Protection are developing recommendations for nutrient management in 89 coastal bays and ponds around Buzzards Bay and Vineyard Sound. Recently they have issued recommendations for limiting the total maximum daily loads (TMDL) of nitrogen to West Falmouth Harbor, Great, Green and Bourne’s Ponds, and subembayments to Waquoit Bay. To meet these TMDLs , significant reductions in nitrate inputs from wastewater and fertilizers will be required. In August of 2006, the wastewater superintendent unveiled a comprehensive wastewater planning strategy (Appendix I). The goal over the next several years will be to develop a plan for increasing the number of properties linked to sewers from the current 3% to nearly 30%. The town anticipates spending about $575,000 to develop this plan. The initial cost estimate for implementation of such a plan is $500 million over 20 years.

We feel that if we can demonstrate that barrier installation to the necessary depth will be practical and that performance will be effective over the long-term, barriers could be a component of Falmouth’s nutrient management plan. Barriers could offer significant cost savings compared with traditional sewering. For example, our commercial partner recently prepared estimates that barriers could be installed along the Seacoast Shores peninsula for between 15% and 30% of the cost of building a sewer collection system. During the next month, we hope to meet with Ms. Lowell and other town representatives to discuss our results to date and these potential cost savings with the goal of ensuring that the barrier approach be considered as one alternative in the comprehensive nutrient management strategy being developed.

Overall Project Timeline Update
Timeline depends on timely return of the MIMS instrument from manufacturer.

Expenditures
Expenditures are in range anticipated for work accomplished.

End User Advisor Feedback
Name: Brian Currie (Town Planner); Amy Lowell (Falmouth Wastewater Superintendent)
Organization: Town of Falmouth
Location: Town Hall Square, Falmouth, MA 02540
Phone number: 508 495 7441 (Currie); 508 495-7341 (Lowell)
E-mail: bacurrie@town.falmouth.ma.us; alowell@town.falmouth.ma.us

1) At this stage, what are the potential applications for this research? Please discuss how you and others could potentially use the technology.
Falmouth’s coastal ponds are significantly impacted by excessive nutrient loading, predominantly from septic systems. One of the town’s highest priorities is to improve the water quality in its ponds. In order to do so, and in order to meet the Total Maximum Daily Load limits currently being established by the Department of Environmental Protection, the town will need to substantially reduce the nutrient load to the ponds. The town is embarking on a comprehensive wastewater and nutrient management planning effort for the watersheds to a number of the Town’s most impacted coastal ponds. Because septic systems are the main source of nitrogen, and because the lower watersheds to these coastal ponds are densely developed, the town is envisioning that large-scale construction of municipal sewers will be required to accomplish the Town’s water protection goals. It has been roughly estimated that municipal sewering could cost the Town approximately $500 million over the next twenty years. Because the water quality problem is urgent and because the costs of municipal sewering are so high, the Town also intends to consider additional nutrient management measures (in addition to municipal sewering) as part of its overall nutrient management plan.

If permeable reactive barriers are demonstrated to be an effective means of removing nitrogen from groundwater, then they (along with other proven alternative technologies) could be considered as a supplement to municipal sewering and/or as an alternative to sewering in portions of the study area. This would of course be particularly appealing if the barriers were demonstrated to be lower cost than sewering but equally or more effective. In addition, because barriers would be underground, passive and relatively unnoticeable when completed, they could have additional appeal as a supplementary and/or alternative solution.

2) What are the key challenges to application of this technology? Please consider the technology itself as well as issues related to regulation, politics, socio-economic pressures, trends in the field etc.
The primary challenge of this technology is the relative lack of data demonstrating its effectiveness in the field, particularly at a large enough scale required in order to be a part of a Town’s overall pond protection strategy.

The researchers identified other potential challenges in their progress report, including the challenge of installing the barrier to sufficient depth to prevent underflow, the need to demonstrate that the barrier nitrate removal is permanent (i.e., due to denitrification, not to sorption or another non-permanent process), the need for more information about the useful life of the barrier materials, potential environmental impacts of carbon release and oxygen removal, and potential impacts of salt water on the barrier’s performance, etc..

Another potential challenge includes property access and environmental issues related to the siting of the barriers. It is my understanding that the barriers would need to be fairly close to the ponds in order to capture groundwater in a location where the “plume” is relatively shallow and thin, and could also need to be fairly long (yet narrow) to intercept a significant fraction of the plume. Excavation in close proximity to the ponds would of course require environmental permitting, and acquiring property easements from multiple property owners along valuable pond front properties could be an additional hurdle.

The fact that the barrier captures nutrients downgradient from their sources is both an advantage and a disadvantage. It is a significant advantage because the barrier would, if successful, address nutrients no matter what their source, whereas sewering for example only addresses septic load, not nitrogen load from fertilizer or atmospheric deposition. The disadvantage is that the barrier could be perceived as solution avoidance if it is not paired with solutions that address nutrients at their source (i.e., sewering, fertilizer reduction, etc.).

However, none of these are necessarily insurmountable hurdles, and further research is warranted to address some of these questions that affect potential implementability.

3) Has anything changed about this project's potential applicability since the last reporting period (not applicable to the first Progress Report)?
The only change is that the town has received additional Total Maximum Daily Load reports from the state, adding to the urgency of the need to address the Town’s nutrient loading problems.

4) Questions/comments/ suggestions for the researchers?
Please refer to response to question 2 above.

PI Response to End User Advisor Feedback
The PI’s are planning to meet with town officials in the immediate future to present current research results and what further research is necessary to convince town officials on the efficacy of PRB’s in removing groundwater nitrate.

References
Brunet,R.C. and Garcia-Gil,L.J. (1996) Sulfide-induced dissimilatory nitrate reduction to ammonia in anaerobic freshwater sediments. FEMS Microbiol.Ecol. 21, (2) 131-138.

Davidson,E.A., Chorover,J., and Dail,D.B. (2003) A mechanism of abiotic immobilization of nitrate in forest ecosystems: the ferrous wheel hypothesis. Global Change Biology 9, (2) 228-236.

Moorhead,D.L., Currie,W.S., Rastetter,E.B., Parton,W.J., and Harmon,M.E. (1999) Climate and litter quality controls on decomposition: An analysis of modeling approaches. Global Biogeochem.Cycles 13, (2) 575-590.