Project Title: Development of an Automated Chemical Sampler/Analyzer for Submarine Groundwater Discharge in Estuaries
Principal Investigator(s): Dr. Matthew Charette, Dr. Edward Sholkovitz

Accomplishments
Scheduled Tasks:
Our primary objective for the second half of proposal Year 1 was to begin preliminary field testing on an instrument integrated from subsystems acquired and tested during the first six months of the project. Also, we have amassed a one-year time series for nitrate and ammonium from various locations in Waquoit Bay and its sub-estuaries.

Progress on Tasks
During the spring of 2001, we began lab testing a novel method for estimating seepage meter flow rates, based primarily on a spectrophotometric technique employing dyes. This method involves the injection of a dye into a mixing chamber attached to the seepage meter. By measuring the drop in absorbance from the initial known concentration of dye, the flow-rate can be calculated. In order to avoid the cost and complexity of a dedicated spectrophotometer, we used the NAS-2E nitrate analyzer to make the absorbance measurements. This has shown promise in the lab (Figures 1a and b) and in limited field testing (Figures 2a and 2b).

Difficulties Encountered
There were two main obstacles encountered during the second half of Year 1. First, we discovered that a significant component of dissolved inorganic nitrogen (DIN) in discharging groundwater exists as ammonium (Figure 3). Since our in situ nutrient analyzer does not detect this nitrogen species, we are exploring the possibility of obtaining (on trade-in) an improved version of the NAS-2E that would allow us to measure both nitrate and ammonium. Second, there is evidence that the mixing chamber from which we make flow rate measurements is restricting flow from the seepage meter (Figure 3). We are currently conducting tests to determine the source of the restriction and if it can be improved with a new chamber design.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
The project is on track as scheduled. The preliminary data has provided us with some exciting results (as highlighted below), and we hope to build upon that success during the next six months.

Preliminary Results
We made significant progress during the reporting period. Specifically, the past six months saw the transition from benchtop testing of an idea (the dye-based flow measurement), to the early stages of field deployment of the instrument. As described above, we constructed a mixing chamber that is attached to a traditional seepage meter in the field. In Figure 1a, we show results from the benchtop testing of the chamber where flow was simulated using a peristaltic pump. The figure shows three consecutive dye injections where the spectrophotometer measured an exponential decrease in absorbance as the chamber water was diluted by incoming flow. The slope of this decrease can be used to determine the flow rate (Figure 1b). Benchtop tests at various flow rates provide us with the information we need to calibrate in situ field data. The experiment shown in Figure 1 illustrates that flow rates as low as 9 ml/min are, in theory, resolvable in the field.

Most recently, we began field testing the dye-based flow rate system at our field site in Waquoit Bay, MA. Figure 2a illustrates the field equivalent of Figure 1a where a dye injection is followed by a series of absorbance measurements. The decrease indicates the mixing chamber is being flushed by groundwater flow through the seepage meter. Converting this data into flow rate revealed a decreasing trend of flow rate with the transition from low tide to an incoming tide (Figure 2b). This trend was expected based on data obtained with traditional Lee-type (manual) seepage meters.

In order to confirm if the automated flow rates were directly comparable with manual measurements, we deployed our automated seepage meter and surrounded it with 6 Lee-type seepage meters. We then collected hourly measurements of flow over a seven-hour time period (Figure 3). For all the meters, flow increased to a maximum then decreased to rates observed at the beginning of the experiment. The increase in flow was correlated with the falling tide. However, the Lee seepage meters measured flow rates were roughly three to five times higher than the dye-based automated seepage meter. We are currently designing a test to see if this problem is related to a restrictions in flow by the mixing chamber or the plumbing associated with the mixing chamber as this is the major difference between the two designs.

Finally, we manually collected nutrient samples from each of the seepage meters and from baywater above them (Figure 4). Dissolved inorganic nitrogen (DIN) concentrations varied with time and ranged from < 1 µM to over 300 µM. In the seepage meters, the fraction of the DIN as ammonium was typically greater than 95%. These results confirm our need for an in situ nutrient analyzer capable of measuring both nitrate and ammonium.

Tasks and activities for next reporting period

Tasks for the next reporting period
Continued field-testing of the prototype and presentation of results to date at a national meeting in November 2001 (Geological Society of America, Boston, MA). Coupling of in situ nutrient analyzer with the automated seepage meter to determine time-series DIN fluxes.

Work plan to accomplish tasks
Most of our efforts will be focused on continued field testing of the instrument to make sure results are comparable with manual seepage meters. Assuming the instrument is performing reliably, we hope to begin the data interpretation phase of the project over the next six months.

Concerns or difficulties
We have a need to reallocate funds budgeted for a multi-port water sampler to obtain a multi-channel in situ nutrient analyzer for measuring both nitrate and ammonium in the discharging groundwater. As the current instrument can only perform nitrate analyses, the new instrument has an additional spectrophotometer that would allow us to determine simultaneously: (1) dye-based flow rate measurements, (2)-dissolved nitrate, and (3)-dissolved ammonium.

Expenditures
Expenditures were in the range anticipated for the work accomplished to date.