WATER BALANCE FROM GEOCHEMICAL TRACERS
Figure 9 shows a simplified schematic of the water sources to Salt Pond. Geochemical methods can be used as tracers for some of these sources as they rely on naturally occurring chemical components dissolved in the water (dissolving from rocks, originating from the ocean and rain). An obvious tracer for ocean water is salinity, but to differentiate seawater entering the pond via beach berm overtopping due to tides and waves (Process 3 on Figure 9) from seawater intrusion in the subsurface (Process 2 on Figure 9) radon is used. Radon can only be acquired if the water spends time in the subsurface flowing through rocks and sediment. It is produced in the rocks and dissolves in groundwater (both fresh and salty). Geochemical tracers like radon help quantify processes 5 and 2 as any water that percolates through the sediments will acquire a unique signature.
Methodology
Radon (isotope 222Rn with a half-life of 3.8 days) was used as a tracer of groundwater inputs into the pond area and to the punas (dug wells) to quantify the subsurface connection these water bodies may have with the surrounding aquifer as well as with the ocean. Such hydrological connections[1] with the ocean, for example, would mean that water from the pond area can drain into the ocean should hydrological gradients allow it. But since the hydrological connection works in both directions, seawater intrusion could fill the pond area with water at high ocean levels (spring and king tides, sea level rise). This technique is able to detect such water flow, for example as groundwater discharge into the punas. To measure groundwater discharge rates into the punas and pond area, a commercially available radon monitor (RAD Aqua, manufactured by Durridge) was used to perform continuous measurements of radon in 15-minute intervals, accompanied by water temperature and salinity measurements.
Time series radon measurements were performed in two puna: Puna 1 on 11/25/19 9:34-16:35 (Lat: 21.8988, Long:-159.6065) and Puna 2 on 11/26/19 9:47-14:17 (Lat: 21.8989, Long: -159.6066). Salinity grab samples were collected in the field, diluted with deionized water and measured using a hand-held refractometer, and temperature was measured in situ every 5 minutes using a CTD Diver (manufactured by Schlumberger).
A spatial survey of radon concentration in the water column was also performed in the flooded area of the pond outside of the salt making area on 11/25/19 10:01-11:56. A RAD-Aqua instrument was mounted on a kayak, which was towed around the pond making a half circle around the salt making area. Salinity grab samples were collected in the field, diluted with deionized water and measured using a hand-held refractometer, and temperature was measured in situ every 5 minutes using a CTD Diver (manufactured by Schlumberger). Another survey was performed along the nearshore (<5 m) coastline to determine groundwater discharge from the pond area into the ocean on 11/26/19 10:09-10:59. The RAD-Aqua with a CTD Diver which recorded salinity and temperature were mounted on a kayak and towed along the shoreline. In addition, a CastAway CTD was used to measure salinity and temperature depth profiles along the survey track (Appendix 3). A Garmin GPS was used to record location and time along the survey tracks.
Groundwater discharge rates required to sustain the measured radon concentrations have been derived from a steady state 222Rn mass balance equation, accounting for sources and sinks of 222Rn: groundwater discharge (Fgw), ingrowth from its parent 226Ra dissolved in the water column (Fprod), diffusion from sediments (Fdiff), radioactive decay (Fdec), and evasion to the atmosphere (Fatm). This can be expressed with the following equation as a balance of 222Rn source and loss terms in the surface water column:
which can be expressed as
where QGw is groundwater discharge (m3/m2/d); CRn_gw,, CRn_sw, CRn_air are measured 222Rn concentrations in groundwater, surface water column, and air, respectively (Bq m-3); CRa_sw is measured 226Ra concentration in the water column (Bq m-3); h is water depth (m); 𝜆226Ra and 𝜆222Rn are the decay constants of the corresponding isotopes; ɑ is Ostwald solubility coefficient and k is gas transfer coefficient (m d-1) which was calculated from wind speed according to MacIntyre 1995[2]. For this study Fdiff and CRa_sw were taken from the literature (Corbett et al. 1999; Dulai et al 2016)[3],[4].
Results
Puna 1 had a depth of 1.3 m, average radon concentration of 3.1±1.6 Bq/m3, temperature of 31.4±1.8 oC, and salinity ~120 ppt (Table 2, Appendix 3). Puna 2 had a depth of 0.97 m, average radon concentration of 4.4±2.2 Bq/m3, temperature of 39.6±0.3 oC, and salinity ~120.
Groundwater discharge rates were 0.01-0.13 m3/d in Puna 1, and 0.02-0.18 m3/d in Puna 2 (Table 2, Appendix 3). These rates are very slow, and assuming a puna volume of 2-3 m3 these groundwater discharge rates would correspond to adding/replacing 4-5% of the puna volume per day.
The pond survey results revealed very low radon concentrations (0-3 Bq/m3, average 0.56 Bq/m3), average water depth of 0.3 m, temperature of 32.7±2.9 oC, and salinity of ~120 ppt (Table 3, Appendix 3). The measured radon levels were mostly corresponding to those supported by passive diffusion without the need for groundwater discharge contribution; only a few locations had radon concentrations high enough to derive groundwater fluxes. Corresponding groundwater discharge rates were 0.004-0.015 m3/m2/d and average discharge for the whole pond area was 0.002 m3/d, suggesting a hydrologically very isolated system with minimal groundwater inputs.
The coastal survey revealed overall low radon concentrations (0-2.8 Bq/m3), except for an area in the NW of the embayment of Salt Pond Beach, where concentrations jumped to 14 Bq/m3. Groundwater discharge rates in the NW corner were ~175 m3/d (Table 3, Appendix 3). This estimate reflects groundwater discharge in the very nearshore region only, where we could expect exchange with the salt pond, if any. More discharge may be present farther offshore, but that was not targeted in this study. In comparison to other coastlines across the State, these nearshore discharge rates are orders of magnitude lower (Maunalua Bay, Oahu, Kiholo Bay, Hawaii, Lahaina, Maui, and Hanalei Bay, Kauai all have typically 1,000 -10,000 m3/d of discharge)[5],[6],[7],[8], so this coastline is hydrologically not highly conductive.
The CTD CastAway salinity and temperature depth profiles only showed minute differences between groundwater impacted and ocean sites. This is due to the small magnitude of groundwater discharge. The site with the highest Rn had a small but noticeable decrease in salinity and temperature (Figure 12).
(Figure 10 – Photograph of the spatial survey researchers in the pond with the kayak and the RAD-Aqua with CTD Diver equipment.)
(Figure 11. Diagram of radon surveys in the pond and in the nearshore ocean that identified locations of groundwater discharge (expressed as m3/d). Time series radon measurement locations in the two puna are indicated by the x. All measurements were done on November 25 and 26, 2019.
(Figure 12. Temperature and salinity depth profiles in the NW part of Salt Pond Beach where the maximum radon was detected (SGD-submarine groundwater discharge) and in the middle of the bay (OCE-ocean conditions). The latter was warmer and had higher salinity (Table 1, Appendix 3).)