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Constraints on Subsurface Density from Gravity Surveying In and Around the Salt Lake Tuff Ring Complex, South‐Central Oahu

Garrett Ito, Justin Higa, Robert Whittier, Niels Grobbe, Nicole Lautze, and Donald Thomas
June 2019, 13 pp.


We report the outcome of a reconnaissance, on‐land gravity survey, in and around the Salt Lake Tuff Complex (SLTRC) of south‐central Oahu, Hawaii. The survey spans an area of ~4500 × ~3500 m and comprises 332 measurements with an average spacing of ~200 m. The data were corrected for the effects of measurement elevation (free‐air correction) and for the gravitational attraction of topography (complete Bouguer correction) using a LiDAR‐derived, digital elevation model (DEM) with 1 m accuracy in all directions. A regional gradient associated with the large scale structure of the Koolau shield volcano was estimated and removed to isolate the local subsurface variations of interest. The resulting residual Bouguer anomaly (RBA) varies by ±2 mGal. The RBA was then inverted to produce solutions for the 3D density structure that fit the RBA to measurement uncertainty. A wide range of plausible solutions were produced by varying the density used for the topographic correction (2300–2700 kg/m3) as well as parameters of the inversion that influence the depth range of the density heterogeneity in the solutions. The 996 solutions show heterogeneity extending to depths of >2 km bsl, and appreciable differences in detailed 3D structure between models. Despite these differences, the planview patterns of mean density anomaly within the depths (~240 m bls to ~6 m above sea level) of the fresh water reservoir are all very similar. The planview patterns differ primarily in total variation: the median is ~180 kg/m3, and 180 kg/m3± 30 kg/m3 represent the 10th and 90th percentiles. The most prominent feature is an RBA and mean density high in the northern Aliamanu Tuff Ring and along its southern border near the center of the SLTRC. To the east, the RBA and mean density anomaly decrease to lowamplitude values, and then increase again to positive values near the base Moanalua Ridge. Around the base of Red Hill Ridge and at the mouth of Moanalua Valley just south of Red Hill Ridge, the RBA and mean density anomaly are negative. A base‐level interpretation of the gravity data postulates that basaltic rock associated with higher RBA also have lower porosity, and correspondingly lower hydraulic conductivity, whereas basaltic rock having negative RBA and lower density contrast may be more hydraulically conductive. Our findings provide a structural framework that can be used with existing and new data to make improved inferences of groundwater flow, or for the design of future geophysical (e.g., seismic or electrical) investigations targeting more detailed structure.

Identifying priority watershed management areas for groundwater recharge protection on Hawai‘i Island

Leah L. Bremer, Nathan DeMaagd, Christopher Wada, and Kimberly M. Burnett
November 2019, 27 pp.


This report provides an analysis of the relative effectiveness of watershed conservation and restoration efforts in terms of groundwater recharge benefits in Hawaiʻi County Department of Water Supply (DWS) priority aquifers and recharge areas. In Kohala, Kona, and Kaʻū. With financial support from DWS and the National Science Foundation, EPSCoR ʻIke Wai project, this study builds upon a previous effort funded by the Hawaiʻi Community Foundation (HCF). Specifically, this report extends the previous report for HCF by: 1) expanding the area of interest to include priority recharge areas as well as target aquifers identified by DWS; 2) including an analysis of potential spread of non-native grassland into native forest areas; 3) including an analysis of changes in potential fog interception with change in land cover; and 4) adding an assessment of priority areas for native forest restoration. Using a combination of land cover and water balance modeling, we created priority maps for watershed protection and restoration based on the highest potential groundwater recharge benefits compared to “no protection” or “no restoration” scenarios. The results of this report are based on best available data at the time of the work. Ongoing data collection by the United States Geological Survey (USGS) and the University of Hawaiʻi at Mānoa to better characterize hydrologic processes associated with different forest types should eventually help to refine these estimates.

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September 2019, xi + 84 pp.


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