Water Resources Research Act Program

Details for Project ID 2014HI434B

Tracing Interconnectivity Between High-level and Basal Groundwaters in West Hawaii

Institute: Hawaii
Year Established: 2014 Start Date: 2014-03-01 End Date: 2016-02-28
Total Federal Funds: $21,422 Total Non-Federal Funds: $43,321

Principal Investigators: Craig Glenn

Abstract: Recent rapid development in the North Kona District of West Hawaii has raised concerns regarding the sustainability of the region’s groundwater resources in the face of increasing demand. A large portion of the region’s municipal water supply is withdrawn from wells tapping a high-level groundwater system, yet the effects of withdrawals from this high-level aquifer on the underlying and adjacent basal groundwater system, which itself sustains the region’s unique coastal ecosystems while at the same time being utilized for municipal, industrial, and irrigation purposes, are not understood. This project is thus designed to demonstrate the degree of hydrologic connectivity between the high-level and basal aquifers within the North Kona (North Keauhou) district of Hawaii. While the abrupt freshening and rapid rise in the elevation of the inland higher-level waters clearly indicates that these strategic aquifers are distinct, the nature of their divide and, importantly, the degree of connectivity between their water masses remains uncertain to completely unknown. Groundwater is withdrawn from both of these sources, but due to dramatic increase in needs for potable supplies, a large and every increasing portion of the region’s municipal water supply is withdrawn from wells tapping the high-level groundwater system (Oki, 1999). However, the effects of withdrawals from the high-level aquifer on the underlying and adjacent seaward basal groundwater system, which itself sustains the region’s unique coastal ecosystems (via salinity and nutrient balance) while also being utilized for municipal, industrial, and irrigation purposes, are not well understood. Additionally, the effects of increased development, including industrial, agricultural, and waste disposal activities on groundwater quality in this area are also poorly known due to both a lack of water quality data, and a lack of understanding of the region’s subsurface hydrology. Recent studies of west Hawaii groundwater (Bauer, 2003; Kelly, 2012) have suggested that the high-level groundwater system in the Kona area does drain into the basal lens and that a substantial portion of coastal basal groundwater originally fell as rain in the upland regions of Hualalai volcano and Huumula Saddle. Yet, recent proposals for land development have implied that relatively impermeable barriers may separate these waters. The research proposed here is needed to test such disparate hypotheses and to fully evaluate the proportion of mixing that occurs between these critical aquifers. We propose to use end-member mixing analysis of the conservative stable isotopes 2H and 18O in precipitation, wells, coastal springs and ocean waters as tracers with which to evaluate the North Kona groundwater connections. Variations in these isotopes in precipitation due to differences in temperature, elevation, and distance from the coast have been used previously in the Hawaii (Scholl et al., 1996; Scholl et al., 2002) and in other parts of the world (e.g. Gat, 1971; Fontes, 1980; Warrier et al, 2012) to constrain groundwater recharge areas, indicate mixing, and delineate different groundwater systems. In order to properly characterize variations in the stable isotopic content of precipitation across a study area as large and climatically diverse as west Hawaii, cumulative precipitation collectors must be deployed for several long (typically 6-month) intervals in locations representing as full a range as possible of temperatures, elevations, and distances from the coast. Anticipating this need, we have designed, deployed and sampled 8 of these collectors in the Kona area since 2011, and are thus now uniquely poised to fully undertake and complete the remaining steps of research needed: Combined with variations in well temperatures and salinity, we will use the results of the water isotopes collected from precipitation, groundwater, springs, and seawater to deconvolve mixing. The determination of δ2H and δ18O values for all sources will provide the means to assess the contribution of high-level groundwater to the basal system via end-member mixing analysis. For the purpose of this analysis, seawater, local precipitation (approximated by the volume weighted average of precipitation collected across the basal aquifer’s elevation gradient), and high-level groundwater will be employed as the end-members. The percent contributions of these three end-members to each basal groundwater sample will then be determined analytically using δ2H and δ18O values as tracers (e.g. Christophersen and Hooper, 1992; Hooper, 2003; Liu et al., 2004; Liu and Kao, 2007). We have had high success in using such combined geochemical–isotopic water partitioning analyses in other settings in Hawaii, the results of this project will provide important new and unique results with which to appraise the water sustainability and coastal issues within the study area, and this work will provide a firm foundation for extending this approach to other settings throughout the State.