CICEET Progress Report for the period 08/01/01 through 01/31/02

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

Accomplishments
Scheduled Tasks:
Our primary objective for the first half of proposal Year 2 was to continue preliminary field-testing and modification of an instrument integrated from subsystems acquired and tested during the first six months of the project. Also, we have amassed an eighteen-month 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 and in limited field-testing.

We conducted two field deployments of the instrument in Waquoit Bay during Fall 2001. The first deployment involved manual dye injection in combination with seepage meter bags to check for flow restriction from the mixing chamber design (Figure 1). The second deployment, which was ~2 days in duration, utilized the NAS-2E for the dye injection and absorbance measurement (Figure 2). Lastly, we developed an algorithm for converting dye absorbance measurements into groundwater seepage rates.

Difficulties Encountered
We are currently addressing the difficulties encountered during the last reporting period and have no new concerns at this time.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
The project is on track as scheduled.

Preliminary Results
The dye injection method has shown enough promise that the continued development of the automated in situ system is justified. Figure 1 illustrates that seepage rates predicted from manual dye injection peaked near low tide (upper box) and were in good agreement with the seepage rate calculated from traditional bag collection (lower box). Both the magnitude of the seepage rate and variation with tidal stage were in excellent agreement with results from our prior CICEET progress report (8/2001).

An automated tests (using the NAS-2E) with the dye injection method have also confirmed that the flux of groundwater is tidally dependent (Figure 2). In addition, this field test demonstrated that the method is capable of measuring flow in both directions. Though there is a net flux of groundwater discharge to Waquoit Bay, there is a measurable amount of seawater infiltration into the sediments during high tide. Reliability of the dye-based method over long deployments (> 3 days) has been an issue. We believe this is related to dye staining of the spectrophotometer. We believe deployment length can be improved using a fluorescent dye; we will report on this in the next update.

The in situ nitrate analyzer had revealed that there are large increases of nitrate in surface waters near the seepage face associated with the increased flux of groundwater at low tide (Figure 3). These results suggest that, rather than via steady diffuse flow, nutrients are tidally pumped into Waquoit Bay, where nitrate concentrations are typically >100 µM in groundwater.

Tasks and activities for next reporting period

Tasks for the next reporting period
Continued field-testing of the prototype leading up to a submarine groundwater discharge (SGD) intercomparison experiment on Long Island, NY in May 2002. There, we hope to showcase our CICEET-funded instrument against other (heat-pulse and ultrasonic) automated seepage meter technologies.

Work plan to accomplish tasks
Most of our efforts will be focused on constructing a new integrated seepage meter funnel and mixing chamber for the SGD intercomparison. 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
None at this time.

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