Submarine Ground-water Sampling

In addition to the EC probe studies described previously, the USGS team used a hydraulic vibratory drilling rig mounted on an all-terrain vehicle (ATV) and secured to a barge (Figure 7) to advance piezometers for submarine ground-water sampling. Sampling was performed at seven sites in Salt Pond, Salt Pond channel, and Salt Pond Bay (Figure 1 and Figure 3) from August 22 to August 25, 2006. Piezometers consisted of 2.5-cm-diameter steel pipe in 3.4-m lengths that were joined by hydraulically crimped couplings. The screen consisted of a shorter section of pipe with two laser-cut vertical slots 0.25 mm wide and 67 cm long on opposite sides, attached to the lead section of riser pipe. Nominal sampling depths reported were measured from the sediment surface to the bottom of the screen. Sampling intervals were generally every 305 cm, except for the first site (SP2005-1), where sampling began at 152-cm intervals.

After advancing to each sampling depth, polyethylene tubing with a small check-valve in the end was inserted into the piezometer pipe and moved up and down by hand to purge muddy water and develop the piezometer. After turbidity decreased, the tubing was connected to a peristaltic pump and field-water-quality measurements of temperature, conductivity, dissolved oxygen, pH, and oxidation-reduction potential were made in a flow-through cell by using a YSI 650 Multi-Parameter Data Sonde (Table 6). Filtered samples were collected for nutrients, radon, radium, and metals analyses after field parameters stabilized (Table 6). Only nutrient data are reported here.

Figure 11: Salinity cross section.

Ground-water salinity values from the piezometer sampling intervals were consistent with surface electrical resistivity data collected in 2004 and 2006 and EC probe measurements (Figure 11). Sampling and probe measurements both indicated that Salt Pond is underlain by brackish ground water in sediments to a depth of about 15 m below the sediment surface. The channel connecting the pond to Nauset Marsh has nearly fresh water at shallow depths (3 m or less below the sediment surface). Salt Pond Bay in Nauset Marsh is underlain by a layer of fresh ground water over 17 m thick as far offshore as measured (about 500 m). Nutrient analyses show that nitrogen is mostly present as ammonium in shallower ground water, but significant nitrate/nitrite is present in one deep sample from beneath Salt Pond (site 4, Figure 12a). No significant increase in nitrate/nitrite with depth was observed at the greatest depth penetrated beneath Nauset Marsh (24.4 m), but an elevated organic nitrogen concentration was present at this depth. The results for the phosphorus species measurements indicate that the highest concentrations are present in brackish to saline samples (Figure 12b).

Figure 12a: Profiles of nitrogen species in ground water.

Figure 12b: Profiles of phosphorus species in ground water.