1. / SUBMARINE GROUNDWATER DISCHARGE (SGD) SEEPMETER MEASUREMENTS: ELIMINATION OF ARTIFACTS AND THE BERNOULLI INFLUENCE B. M. Mwashote & W. C. Burnett, Department of Oceanography, Florida State University Fig. 2: Study area showing locations of seepmeters (modified from Taniguchi et al. 2003) REFERENCES Burnett, W. C., Bokuwniewicz, H., Huettel, M., Moore, W. S. and Taniguchi, M. 2003. Groundwater and pore water inputs to the coastal zone. Biogeochemistry, 66(1-2), 3-33 Huettel, M., Ziebis, W.,and Forster, S. 1996. Flow-induceduptake of particulatematter in permeablesediments. Limnology and Oceanography, 41(2), 309-322. Lee, D. R. 1977. A device for measuring seepage flux in lakes and estuaries. Limnology and Oceanography 22 (1), 140-147 FUTURE RESEARCH The next phase of research work is to apply the seepmeters in other nearshore coastal environments and in the determination of nutrient budgets associated with SGD ABSTRACT Recent SGD data obtained from experiments conducted using conventional seepage meters (Lee type seepmeters), placed in areas exposed to currents, waves, winds and ocean swells, have been found to be subject to varying degrees of artifacts compared to those placed in relatively calmer environments. The observed artifacts are mainly associated with Bernoulli-induced flow, the vertically directed flow arising due to wave and current movement across topographic features. In an attempt to address this apparent limitation in SGD measurements using this technique, experiments conducted at the northeastern portion of the Gulf of Mexico, next to Florida State University Coastal and Marine Laboratory (FSUCML) using automated heat-type seepmeters (modified version of the Lee type seepmeters), have shown that it is possible to effectively eliminate or reduce these artifacts by simply placing the seepmeter at nearly the same level as the sediment topography. Where the submergence of a benthic chamber is less feasible in the sediment, a viable alternative that proved effective in circumventing SGD artifacts is to deploy in parallel, an identical seepmeter “blank benthic chamber” inside a non porous material such as a child’s plastic play-pool, as a control. Measurements obtained with this approach compared reasonably well with geochemical tracer techniques. INTRODUCTION Seepage meters (seepmeters) have been used to quantify SGD in a variety of conditions in many nearshore coastal environments and lakes over the years (Lee 1977; Bokuniewicz 1992; Cable et al. 1997). However, these devices have recently drawn concerns with regard to the Bernoulli-type flow induced around the seepmeters (Shum 1992; 1993). This was suggested to be an important influence in a study conducted at Florida Keys (Shin et al. 2003). Several other studies at the same area have gathered strong evidence that show a link between tidal and aquifer head variations and seepage measurements (Chanton et al. 2003; Corbet and Cabble 2003). Various variables that may influence the observed rate of SGD include hydraulic gradients, sediment permeability, mound height, ripple length and amplitude, wavelength and water wave period. It has recently been demonstrated that water penetration into sediments results from pressure gradient that develops over the sediment bedforms (Huettel and Gust 1992; Huettel et al. 1996). In their study conducted in sandy ripple bed, Huettel et al. (1996) showed that differential pressure can result to vertical pore water velocities as high as 3cmh-1 (72cmday-1) in 2 to 3cm high when bottom water current velocities approach 10cms-1. In a more recent study at a site in the eastern coast of Florida, Cable et al. (2006) found Bernoulli-induced flow to be real, but maybe minor component of the physical forcing mechanisms that may influence seepage measurements from sediments. It is in view of the foregoing background knowledge, that the present study was designed and conducted in order to assess the influence of Bernoulli-related flow artifacts in SGD seepmeter measurements, and the subsequent testing of possible practical ways of eliminating these artifacts so as to enable the seepmeters remain viable tools in SGD measurements. MATERIALS AND METHODS Study area Field measurements were carried out at a study area located in the northeastern coastal Gulf of Mexico, an area known for the presence of seepage and submarine springs (Rutkowski et al. 1999; Burnett and Dulaiova 2003). The area (Fig. 1) lies within the Woodville Karst Plain, which extends from about 80km inland to the coastal zone where the Florida State University Coastal and Marine Laboratory (FSUCML) is located at Turkey Point, Florida. The horizontal hydraulic conductivity of the unconfined aquifer in the study area is estimated to range from 2 x 10-4 (3m depth) to 6 x 10-3 (0.5m) cms-1 from slug tests (Rasmussen et al. 2003). A more detailed description of the study area is given elsewhere (Bugna et al. 1996; Taniguchi et al. 2003). Field measurements The automated heat-type seepage meters (seepmeters) used in this study were modified from Lee (1977). The seepmeters (Fig. 2) were allowed to equilibrate for at least 24h before measurements started, with a datalogger programmed to allow automatic readings to be made at a 2 minute interval, and storage at 10 minutes intervals, for duration of days at every field deployment. During any particular deployment, up to five automated seepmeters were deployed at the study site, facilitated by aid of a special 76.2m (250 ft) long electrical extension cable for each seepmeter, transect-wise from the shore toward the sea (Fig. 2). The furthest distance of deployment of the seepmeters from the shore was 182m, while the shortest was 15m. The datalogger, battery and all associated accessory assembly was located and operated from the shore. This arrangement offers a robust set up with considerable advantages, especially for long-term deployments. In order to asses the effect of Bernoulli – induced flow in seepmeter measurements, two seepmeters were deployed side by side at the study site, with one of the benthic chambers completely submerged or nearly the same level as the sediment topography, and the other partially submerged, leaving a short length of ~5-10cm exposed. Parallel seepmeters that acted as “blank” controls were similarly deployed inside non-porous materials (such as a child’s plastic play-pool) at the same sites. In addition, comparative experiments were also conducted to compare continuous heat-type seepmeter SGD measurements with those obtained through manual (or Lee- type) seepmeters (Figure 3). During the same period, radon, which is a good SGD geochemical tracer, was also monitored at the same study site by employing two RAD7’s (Durridge Co. Inc.) according to the procedure described in Burnett and Dulaiova (2003). RESULTS AND DISCUSSION While important environmental factors, including water level (tide), wind, hydraulic gradient, temperature, wave set up, swells and rainfall, are important considerations in SGD measurements, it is likely that these environmental factors impact different SGD measuring techniques to different extents (Figure 4 and 5). It has been nonetheless observed that most of these conditions combine to induce pressure gradients that generally lead to Bernoulli-type flow artifacts in seepmeters (Huettel et al. 1996; Shinn et al. 2002; Cable et al. 2006). From the results shown in Figure 4, it is evident that wind speed and direction can play an important role in influencing the magnitude of SGD measured using both seepmeter and radiochemical tracer techniques. These environmental factors need to be appropriately taken into account if reasonable SGD estimates are to be obtained through these techniques. Our results have demonstrated that submerging the seepmeter benthic chambers at least up to the same topographic level of the sediment, effectively eliminates or reduces the effects of the Bernoulli-induced flow that appears to be strongly enhanced by wind and tide action (Figure 5). Alternatively, parallel seepmeters could be similarly deployed inside non-porous materials (such as a child’s plastic play-pool) to act as “blanks,” and then subsequently accounted for. When data obtained in this study is compared to other similar studies (Table 1), it is evident that our seepmeter SGD measurements are in close agreement with other independent SGD measuring techniques that have been employed at the same study area, and elsewhere. CONCLUSIONS The continuos heat-type automated seepmeter is a suitable device that has the potential of providing reasonably consistent and reliable SGD measurements, and can be applicable for long term SGD study in most nearshore coastal environments. Submrging the seepmeter benthic chambers completely or to the same topographic level of the sediment during SGD measurements, effectively eliminates Bernoulli – induced flow that appears to be strongly enhanced by wind and tide action. However, in the event of impracticality of benthic chamber submergence in the sediment, “blank seepmeters” deployed in parallel, can be used to serve the same purpose. Seepmeter measurements provided data that agreed closely with other SGD rate measuring techniques including the radon geochemical tracer.