CICEET Progress Report for the period 10/01/04 Through 3/15/05

Project Title: Use of Permeable Reactive Barriers to Reduce the Release of Nitrate from Existing Septic Systems to Groundwater and Estuaries
Principal Investigator(s): Nancy E. Kinner, Thomas P. Ballestero, Robert Roseen

Figures

Figure 1

Tables

Table 1

Table 2

Progress on Tasks
1. Laboratory Column Experiments
At the time of the last progress report, the laboratory column experiments had been completed. The column data indicated that denitrification occurred following first order kinetics with a half life (t_1/2) for nitrate of 1.5 to 3 days for pine shavings and wood pellets (as compared to a blank which exhibited no significant nitrate removal).

2. Pilot Scale Field Monitoring
At the time of the last progress report, denitrification had not been observed at the JEL field site, in spite of the fact that the PRB wells were packed with pine shavings and nitrate concentrations in the groundwater were high. This trend continued into the current reporting period covered by this progress report. A bromide tracer study had indicated that the direction of groundwater flow was similar to what we had originally estimated. Denitrification was likely limited by either: (1) the availability of organic carbon (DOC) from the amendment, (2) the low pH of the JEL groundwater (4-5), and/or (3) the relatively high dissolved oxygen (DO) concentration in the groundwater (DO = 3-4 mg/L). Both low pH and high DO have been cited in the literature as inhibitors of denitrification.

3. Laboratory Microcosms
To determine the influence of DOC, pH and DO, a series of laboratory microcosms were performed during the previous reporting period using JEL groundwater. The microcosms indicated that the subsurface microbial community from JEL was capable of denitrifying the laboratory’s septic tank waste when it is discharged into the leachfield provided adequate DOC is present. The microcosms were incubated for as long as 20 days, but the data indicated that performing a test over a shorter period could be useful to determine if the t_1/2 was in the range of 1.2 to 3 days.

Difficulties Encountered
A final microcosm was successfully completed during this reporting period and supported the previous microcosm data, indicating that denitrification was probably DOC limited. These results indicated that either the DOC from the pine shavings could not be used by the denitrifying bacteria in the JEL groundwater or that there was a problem with the supply (diffusion) of the DOC from the PRBs packed with the shavings to the groundwater flowing past/through them. The diffusion issue was the suspected problem because the laboratory columns had shown that the pine shavings provided DOC that could be used for denitrification. This conclusion prompted the addition of a labile, dissolved organic carbon amendment into one of the PRB wells at the JEL site. The organic carbon amendment was sodium lactate (lactate), a well-documented electron donor for denitrification and one that could be added as a liquid into the PRB, limiting the possibility of diffusion limitations from the pine shavings. Lactate had been used in the laboratory microcosms and supported denitrification, so it was the likely choice for the JEL injection.

The lactate injection at the JEL resulted in rapid denitrification, in spite of the fact that the test was conducted during very cold weather in January and February 2005. This supported the conclusion that denitrification at the JEL site had been limited by diffusion of the DOC from the PRBs packed with the pine shavings into the nitrate-rich groundwater.

Subsequently, a PRB packed much more loosely with pine shavings was inserted into the JEL site. Denitrification was observed in the surrounding wells indicating that the PRB packing must be less dense to allow diffusion.

The biggest problem that we faced this reporting period was conducting DOC analyses because the lab’s Shimadzu TOC analyzer was broken. It required several site visits over 3 months by the Shimadzu technician, in addition to many hours of troubleshooting by UNH staff before it was determined that the electronic control board needed to be replaced. The technician had never observed the type of problem that the instrument was having. The instrument is still not totally repaired and this has created a backlog of 300 DOC samples for the PRB project. As a result, this report does not contain DOC data for the lactate injection or the subsequent pine shaving PRB experiment. We hope to have the problem fixed, but it may require purchasing a new instrument (the current one is 10+ years old).

Anticipated Success in Meeting Project Objectives
The column studies, laboratory microcosms and JEL field data have all now shown that denitrification of the groundwater is possible using PRB wells downgradient of a leachfield. The PRB t_1/2 is now being determined in the field at JEL. This work should be finished in late June 2005. A final report will be submitted to CICEET by August 31, 2005, the end date of the project.

Preliminary Data
1. QC
We adhere to published QA/QC requirements as outlined in the original proposal. Percent recoveries for the calibration standards to date have been within 90-110%. The relative percent difference between duplicates has been within +/- 10%. These meet the QC requirements.

2. Microcosm Results
The final microcosm was conducted in November 2004 over a 14 day period (See Table 1). The results supported those achieved during the previous reporting period. The non-DOC amended JEL groundwater had low pH (4-5), with low DO (<0.1 ­ 0.2 mg/L), only ambient, relatively non-labile DOC (5-11 mg/L) and exhibited no denitrification. This mimicked the in situ conditions observed at the JEL field site. The amended microcosms all had initial DOC concentrations in the 75-80 mg/L range. In those DOC amended microcosms where the ambient pH was used (non-buffered, pH = 4), denitrification occurred within 7 days, regardless of whether DO was initially present. It appears that DO was rapidly used as an electron acceptor and therefore its presence did not inhibit denitrification. The increase in pH observed in both cases was not unexpected because denitrification produces hydroxide, which can increase pH in the water if the system is not well buffered. This increase in pH during denitrification did not inhibit nitrate removal. However, those microcosms where the pH was high initially (pH=8) did not undergo denitrification, but did degrade DOC. It is possible that the higher initial pH, in the absence of DO, favored sulfate reducing bacteria even though this process is energetically less favorable than denitrification. Sulfate is usually present in sanitary wastewater and sulfate reducers can function well at high pH.

Overall, the microcosm results indicated that if a labile source of DOC was available in the JEL groundwater, denitrification occur even if the initial DO is high (3-5 mg/L) and the pH is in the range of 4-5. A denitrification rate could not be determined for the microcosms because there was no nitrate remaining at the 7 day test interval. However, the results indicated that the column-estimated denitrification rates (t_1/2 = 1.5-3 days) could be possible.

3. Pilot Scale Field Monitoring
The microcosm results indicated that the problem with the denitrification at the JEL site was clearly related to DOC availability. In order to verify that denitrification was possible in the field, we decided to add a sodium lactate solution into one of the PRBs at the JEL site. A concentrated lactate solution (~500 mg/L) was added to PRB-2 (See Figure 1). PRB-2 was selected because of its proximity to MW-16 and MW-17, monitoring wells located downgradient. The 500 mg/L solution was added to create a 2 ft radius of influence around PRB-2 and a desired in situ DOC concentration of ~ 250 mg/L in the groundwater in this region. We reasoned that if the addition of the dissolved lactate solution resulted in denitrification in the JEL groundwater, as the results of the microcosm indicated it would, the problem was likely the availability of DOC from the PRB pine shavings.

Lactate was added to PRB-2 on January 16, 2005 (after background groundwater monitoring indicated that denitrification was still not occurring in situ. The nitrate decreased relatively rapidly in all of the surrounding upgradient and downgradient MW wells within 2 ft. We look forward to obtaining the DOC results to confirm that the organic carbon concentration increased concurrently in these wells. The pH and the DO decreased somewhat in MW-16 and MW-17 and significantly in the PRB, with a lesser effect in the upgradient wells. pH did not increase as rapidly as it did in the microcosms, possibly because there was better buffering in situ. When the lactate addition was stopped in early February, the nitrate concentrations returned to pre-injection levels as denitrification halted.

The lactate field results supported the conclusion that a soluble source of DOC could foster in situ denitrification using a PRB. The problem with using a dissolved source of DOC, and in particular lactate, is the need for frequent preparation of the solution to prevent microbial contamination and degradation of the DOC in the source container. If the lactate concentrate is made in large batches and held for several days prior to injection, it could be biodegraded so that the solution injected is just water. In addition to the issues of preparation and handling which homeowners will be reluctant to perform frequently, the cost of sodium lactate is high and it is difficult to dissolve because it comes as a very viscous 60% stock (e.g., thick syrup).

Our original concept of using wood products, in particular pine shavings, as a DOC source centered on their relative availability at a low cost (i.e., they are a waste product from wood product manufacturers), their ability to produce DOC that denitrifiers can use, and their ease of use (they are light and easy to pack into the PRBs).

We concluded that the problem with the pine shavings was that they were packed too tightly into the PRBs. Originally, we packed the PRBs tightly with pine shavings to supply a large amount of DOC per unit volume, but in doing so we prevented the DOC from diffusing out of the PRBs into the groundwater. Therefore, we repacked PRB-2 with pine shavings much more loosely and placed it in situ on April 9, 2005, prior to sampling. Immediately, denitrification was observed in the PRB itself and the closest well downgradient (MW-16) (See Table 2). Subsequently, denitrification was also observed in MW-1, MW-2 and MW-17. It is likely that denitrification occurred so rapidly because a population of denitrifiers was already present as a result of the earlier lactate injection. After 11 days, nitrate began to reappear in MW-16 and by 17 days, nitrate had increased to pre-amendment levels in all of the MW wells except the PRB. Once again, it will be good to have the DOC data to confirm the denitrification pattern. It appears that the loosely packed pine shavings are exhausted within 17 days.

The results of this field experiment indicated that the problem with our concept was that the pine shavings were packed too densely into the PRB inhibiting release of the DOC into the surrounding groundwater. Once this problem is addressed, denitrification is readily achieved.

We have removed PRB-2 and loosely repacked it with pine shavings after removing the spent material. We are monitoring the results in the surrounding MWs. If, as we expect, denitrification occurs, we will monitor it over time to determine if the 11- 17 day depletion of the amendment is verified. We will then replenish all of the PRBs at the JEL site, packing them loosely with the pine shavings and conducting a final monitoring event to verify the ability of the PRB network to denitrify the JEL groundwater.

Using these results, we will write the final report and submit it to CICEET. The report will include a discussion of how PRB wells packed with pine shavings could be used in residential leachfields.

Tasks and Activities for Next Reporting Period

Tasks for the Next Reporting Period
During the period from May 1 to August 31, 2005, we will complete the denitrification studies at the JEL PRB field site, adding less densely packed pine shavings to the wells. We will monitor DO, pH, DOC and nitrogen species in the PRBs and upgradient and downgradient wells. We will calculate field denitrification rates and write the final report for submission on or before August 31, 2005.

Work Plan to Accomplish Tasks
We will be conducting the same analytical procedures as outlined in the proposal for the analyses of the field samples. We will also analyze the backlog of DOC samples after the TOC analyzer is repaired.

Concerns or Difficulties
At the present time, we do not anticipate any difficulties completing the project and submitting the final report on time.

Expenditures
The project is within budget at this time. There have been no unusual expenditures.