DISCUSSION
To better understand hydrology in the area, various research techniques were used including electrical resistivity, analysis of radon concentrations, self-potential tests, hydrologic modeling, and analysis of salt pond water level and salinity changes.
Findings indicate that thick (~9 ft) clay layers limit the rate at which water can flow through the ground surface. These geologic features restrict infiltration, which in turn facilitates evaporation and the concentration of salts. This process is highly important culturally; however, it is increasingly being hampered by pressures associated with climate change, such as rising sea level and unseasonal storms, as well as human-induced factors like land use conflicts and contamination.
The thickness of the clay geology in the salt pond was assessed through electrical resistivity testing, which revealed that the somewhat impervious surficial clay layer is approximately 3-meter thick. Overall, it was found that inflows into the salt ponds originate from various sources including from rainfall, surface runoff, ocean water overtopping, groundwater discharge and subsurface interaction with ocean water. However, findings indicate that subsurface interactions are limited owing to the low hydraulic conductivity of the clay geology featured in the area that restricts subsurface water flow. This means that there is not a significant amount of water flowing into the system subsurface from either a terrestrial source or from the ocean. These findings are reinforced by the self-potential study which indicate minimal hydrologic exchange occurring between the salt pond and the ocean waters and by the negligible change in salinity observed in the pond.
The extremely low hydraulic conductivity is a unique feature of the area, and facilitates the formation of salts through evaporation as waters are not able to quickly infiltrate into the subsurface. In fact, it was found through radon analyses that the clay geology in the area results in extremely low groundwater discharge rates compared to other coastlines in the State. Water from the puna, which is physically transferred by salt-makers to the secondary wells and salt beds, is itself extremely salty, which is another unique aspect of this site. Salinity measurements reveal that the clay features contain highly saline water, measuring more than 60 parts per thousand (ppt). To put this into perspective, ocean water typically has a salinity of approximately 35 ppt. The saltier waters contain a higher concentration of evaporites, which crystallize during evaporation. Following the various studies, it remains unclear why the puna waters are so saline. A “salt shelf” embedded between two clay layers about 2-3 feet deep was pointed out by salt practitioners and observed in the field by the researchers. No evidence of such a feature has been found in push core records nearby at the airport, suggesting that this would be a unique feature to the pond. Resistivity measurements indicate that the underlying layers beneath the clay surface material could potentially consist of sand or basalt filled with seawater.
During rainfall and wave overtopping events, the concentrations of evaporites in the ponds become diluted. Such events are known to interrupt the process of salt formation and harvest. Evidence and modeling indicate that the salt-making process can be disrupted for several months following these events. Owing to the presence of low conductivity soils, dilution is not predominantly caused by subsurface flows, verifying that the dilution is instead induced by surface runoff and overtopping. However, modeling suggests that elevated sea level of as little as 0.6 meters may elevate groundwater such that salt-making could potentially be impacted by near-constant dilution. Past observations suggest that present-day evaporation rates in the area are roughly an inch per week during active salt making activity[27]. Evaporation is likely a significant driver of recovery following flooding events.
Long periods of uninterrupted evaporation generally occur in Kauaʻi during the summer months. Kauaʻi generally experiences two main seasons: a dry season and a wet season. The dry season typically occurs from May to October, while the wet season typically occurs from November to April. Climate patterns can vary over time, and changes in seasonality and rainfall patterns can occur due to various factors such as climate change and natural climate variability. It’s possible that there may be some shifts in the timing or intensity of Kauaʻi’s wet and dry seasons, as is the case with many regions around the world. At this point it remains unclear how changing seasonality may impact salt-making at this site. Thus, long term data collection by salt practitioners represents an opportunity to better understand changes in seasonality such that the salt-making harvest can be adapted appropriately if needed. Relationships between the salt-making community and local meteorological agencies and climate researchers would also be beneficial for the continued success of salt-making in the area.