Baseline Water-Quality Sampling to Infer Nutrient and Contaminant Sources at Kaloko-Honokōhau National Historical Park, Island of Hawai‘i, 2009

Abstract

Baseline water-quality sampling was conducted for dissolved nutrients and for chemical and isotopic tracers at Kaloko-Honokōhau National Historical Park on the Island of Hawai‘i. Existing and future urbanization in the surrounding areas have the potential to affect water quality in the Park, and so the National Park Service and the U.S. Geological Survey designed a water-sampling strategy to document baseline conditions against which future changes can be compared. Sites in and near the Park were sampled twice, in July and December 2009, and included four anchialine pools, two large fishponds, five monitoring wells, an upland production well, tap water, and a holding pond for golf-course irrigation water. Water samples within the coastal park were brackish, ranging in salinity from 15 to 67 percent seawater. Samples were analyzed for dissolved inorganic nutrients (nitrogen and phosphorus), stable isotopes (nitrogen and oxygen in dissolved nitrate; hydrogen and oxygen in the water molecule), pharmaceuticals, wastewater compounds, and volatile organic compounds. A case of acute, but temporary, fertilizer contamination was evident along the Park’s north boundary during the turf grow-in period of a newly constructed golf course. A maximum nitrogen concentration 280 percent above background level was measured in monitoring well MW401 in July, later falling to 109 percent above background by December. Two nearby sites (MW400 and AP 144) had nitrogen concentrations that were elevated compared to remaining sites but less severely than at MW401. Aside from this localized fertilizer influence, other water samples had lower nutrient enrichments: 40 percent or less above background for nitrogen and 57 percent or less above background for phosphorus. Background was defined in this study by a graphical mixing line between saltwater from a deep well in the Park and freshwater at a reference well in the mountainous uplands (Honokōhau production well, at 1,675 ft altitude). Potential nutrient sources between the uplands and coastal lowlands that could contribute to enrichment include rock weathering, natural vegetation, fertilizers, septic leachate, and atmospheric deposition – including motor-vehicle exhaust.

Some fraction of septic leachate is expected in groundwater because there are unsewered suburban and commercial developments upslope from the Park that rely on cesspools and septic systems. However, stable isotopes of nitrate did not implicate septic-waste nitrogen (heavy δ¹⁵N values on the order of +10 per mil or greater). Instead, the heaviest δ¹⁵N values of +6 to +8 per mil were associated with the large fishponds, likely as a result of biotic cycling. Water samples with δ¹⁵N values of +3 to +5 per mil were still isotopically heavier than the upland groundwater value of +2 per mil and likely reflect addition of heavier nitrogen, possibly from septic sources, nitrogen-fixing vegetation, or vehicular exhaust. Pharmaceuticals, wastewater compounds, and volatile organic compounds indicated that if contamination is present, it appears to be at low, barely detectable, levels—at least as reflected by the results of this study. The most diagnostic indication of septic contamination was at monitoring well KAHO 2, closest to Kaloko Light Industrial Park, where three pharmaceuticals (carbamazepine, sulfamethoxazole, and thiobendazole) were detected at trace-level parts-per-trillion concentrations. A screening-level test for laundry fabric brighteners indicated positive detection at most sites; however, readings were quite low and if laundry graywater is present, it appears to be a small, dilute fraction. Because the weather was persistently dry throughout the study period, the USGS team was unable to conduct a wet-weather “storm” sampling. Wet-weather results are expected to differ from those reported here, though by how much remains unknown.

First posted January 13, 2015