CICEET Progress Report for the period 3/01/04 through 9/01/04

Project Title: Automated Radon-222 Mapping in the Coastal Zone for Assessment of Submarine Groundwater Discharge
Principal Investigator(s): William C. Burnett

Accomplishments
Scheduled Tasks:
The following activities were scheduled for this reporting interval: (1) finish calibration of the radon preconcentrator; (2) integration of other systems into the multi-detector radon system; and (3) additional field-testing of the CICEET multi-detector radon system.

Progress on Tasks
We have had good progress on all the tasks mentioned above although there continue to be some problems with the preconcentrator system. We have changed the configuration of the plumbing to/from the preconcentrator and that has helped (see below). The multi-detector system continues to perform very well in field tests and we have had good progress on integrating other sensors into the system. The multi-detector system has become an important part of our research effort.

Difficulties Encountered
As mentioned previously (see Fig. 2 from the last report), we have noticed that the gain of the preconcentrator apparently varies with the ambient radon concentration (it should be constant). We have since discovered that our use of laboratory tap water (high in radon as it is groundwater) for testing resulted in concentrating so much radon that we exceeded the linear range of the measurement device (Durridge RAD-7). Another means of testing was developed to produce waters comparable to coastal ocean waters and the results were much improved.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
We are now at the end of the project period and the project results have been exceeded. We originally intended to develop one system (the “preconcentrator”) and we now have a working prototype. A few problems remain but the system does amplify the signal from radon in water by about a factor of 30. The second system, which we refer to as the “equilibrium” approach, is far simpler and has been very successful in its intended use, i.e., measuring radon continuously in coastal ocean waters.

Preliminary Data
As we have done previously, the results for this report will be presented in two parts: (1) the status of the "preconcentrator system," a totally new technology; and (2) the “equilibrium system,” a new and improved approach to existing technology.

Radon Preconcentrator System
This version of the radon in seawater analysis system consists of: (1) a high-flow and single-pass exchanger where seawater enters and radon is de-emanated into a circulating air stream for analysis; and (2) a “preconcentrator system” that will consist of several driers to remove humidity and charcoal traps to concentrate the radon. The radon will then be automatically released to an atmospheric radon analyzer (Durridge RAD-7) for rapid measurement. In order to solve the problem mentioned above concerning count rates that were too high, we developed an alternative to using tap water for testing in the laboratory. Instead, we now use radium-free deionized water in a large reservoir aged long enough so that the radon in water is in equilibrium with the radon in air. This results in much lower (~100 fold) concentrations than in our tap water.

We also modified the airflow through the system (Figure 1). Water enters the exchanger where radon is degassed, air is drawn in an open loop through the exchanger, first to a cold trap to eliminate much of the moisture, then through two Drierite columns to remove remaining moisture, and finally to the charcoal traps for concentration. In between the last drying column and the charcoal trap is a bypass that bleeds off a small amount of air (~0.1 L/min) for continuous measurement of the ambient radon by one RAD-7. This is for testing purposes only. The 2nd RAD-7 in the setup measures the radon after preconcentration and additional drying.

Because of the preconcentration effect, the count rate in the 2nd RAD-7 is much higher (by approximately a factor of 30) producing much lower counting uncertainties. This is illustrated in the results from a recent run that compares the result from the preconcentrator to results from an equilibrium system, an exchanger hooked up directly to a RAD-7 analyzer (Figure 2). The plot clearly shows that the counting errors from the standard system are much larger than those from the preconcentrator.

Equilibrium Radon System
We have described in our last few reports the multi-detector equilibrium system that we designed that allows us to triple the data flow by connecting three RAD-7 detection systems in parallel to one air-water exchanger. By running the boat slowly (3-5 knots) and keeping good records of the GPS position and depth, we are able to produce a detailed radon map along a shoreline. We have now expanded this system by scaling up to six detectors and adding a temperature/conductivity sensor and GPS/depth sounding capabilities. With six detectors, we now are able to run in 5-minute intervals for most coastal systems. This has increased our average data output from about 1 measurement per hour (pre-CICEET) to 12 data points per hour.

During the current reporting period, we conducted field tests and surveys in Biscaye Bay, the Suwannee River estuary, and Sarasota Bay ­ all coastal sites in Florida. The work in Biscaye Bay was done in cooperation with the USGS who have a project there on submarine groundwater discharge. That survey was very successful and follow up studies at some “hot spots” have already been planned. Our survey on the lower Suwannee River and estuary was run July 29-30, 2004. We anticipated higher radon several miles up the river from its mouth because of the presence of large springs (e.g., Manatee Springs) that discharge groundwater into the river in that area. Radon is a good tracer for groundwater because it is greatly enriched in groundwater relative to surface waters and is conservative. The very high radon concentrations in the lower river are clearly influenced by groundwater discharges (Figure 3).

Another survey was performed in Sarasota Bay during August 18-19, 2004. We had done a survey of the entire bay last year and detected a few very high areas in both the north and south areas of the bay. The radon concentrations were amongst the highest we have seen in an estuarine environment (up to >60 dpm/L). We focused this time on these hotspots to confirm the high concentrations and evaluate any possible relationship to salinity. One of these areas is located just off New College in the northern part of the bay (Figure 4). Our results confirmed the high readings noted last year and showed that the high radon occurs in a narrow band along the shore over a few kilometer distance. The salinities in this area did not show any distinct pattern with respect to radon indicating that the groundwater discharge is composed mainly of recirculated bay water. Seepage meter measurements in the same area that had been performed earlier confirmed high rates of seepage of salty water.

An area in southern Sarasota Bay was also investigated and again the high readings measured last year were confirmed. In this case, however, there does appear to be measurable freshing of bay waters by groundwater inputs. A plot of radon versus salinity for the southern area of the bay (Figure 5), while complex because of possible multiple inputs of both radon and fresh water, does indicate that there is a trend of reduced salinity at higher radon concentrations. This supports the concept that the very high radon concentrations in Sarasota Bay are a result of groundwater inputs.

Tasks and activities for the next reporting period

Tasks for the next reporting period
The project period ended on August 31 so this will be the last bi-annual progress report. A final report will be submitted by November 30, 2004.

Work plan to accomplish tasks
The experimental work is completed.

Concerns or difficulties
We have worked with Durridge on development of a data transfer and interface system for the multi-detector radon system and associated sensors. The data transfer is intended to be via wireless links with a hard-wire back up. The system consists of 10 wireless RS232 links for 6 RAD-7 radon monitors, one temperature probe, and 3 additional sensors. This system has been completed but there are some software difficulties remaining before it can operate at full capability.

Expenditures
Expenditures for this project have been in the range anticipated for the work accomplished to date.