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Potential Effects of Roadside Dry Wells on Groundwater Quality in the Island of Hawaii
A large number of roadside dry wells maintained by the County of Hawaii Department of Public Works (DPW) are used to dispose urban storm-water runoff. Concern has been raised that such dry-well use constitutes a contamination source for receiving waters, such as groundwater used for drinking water sources or coastal waters with sensitive environments. The DPW wants to assess whether the dry wells actually pose a contamination threat before undertaking efforts to install and maintain filters in dry wells.
The overall objective of this study is to assess the potential for DPW roadside dry wells to deliver contaminants to nearshore areas or drinking-water wells. The study is proposed in two phases. The objective of phase one is to provide an island-wide inventory of DPW dry wells, determine the drainage area of each, and rank them on the basis of criteria that pertain to their potential to contaminate receiving waters. Wells that have the highest potential to contaminate receiving waters would be candidates for further study in phase two. The objective of phase two is to assess the nature of contaminants and the flow to receiving waters for selected dry wells.
Relevance and Benefits
The results from this study are necessary for the proper management of the water resources in the State, provides information to the public and decision makers about the status of its freshwater resources and how they are changing, and forecasts likely outcomes for water availability, water quality, and aquatic ecosystem health.
Storm-water runoff from urban areas has been recognized as a source of contamination to receiving water bodies. There is concern that the practice of constructing dry wells to reduce roadside flooding will provide direct entry of contaminants to receiving groundwater bodies. This concern was raised by the Kaloko-Honokohau National Historical Park in Kona, Hawaii, when a developer was granted a rezoning request. The County of Hawaii is interested in determining if their dry wells pose a significant threat to receiving water bodies. This study will provide information needed to evaluate the potential effects of roadside dry wells on groundwater quality in the Island of Hawaii.
This project has two phases. Results of both phases will be published in the USGS Scientific Investigations Report series and made available on the internet.
In phase 1, an inventory of all DPW dry wells will be compiled and sorted on the basis of: (1) presence of urbanized land in drainage area, (2) height above the water table and (3) distance to receiving waters such as the coast or drinking-water wells.
In phase 2, numerical model simulations of groundwater flow and solute transport will be used to assess how contamination entering dry wells will be attenuated as it passes through the unsaturated and saturated aquifer. The numerical models will be semi-generic – they will be three dimensional rectangular models that have aquifer properties and conditions that span a range that is characteristic for the Island of Hawaii, but the models will not represent any specific location.
Phase 1 was completed in 2009 and the results were published in USGS Scientific Investigations Report 2009-5249. Numerical groundwater models for phase 2 were constructed and used to assess how contaminant concentrations change as they migrate through the groundwater system from dry wells to receiving waters.
Izuka, S.K., Senter, C.A., and Johnson, A.G., 2009, Reconnaissance assessment of the potential for roadside dry wells to affect water quality on the Island of Hawai'i: U.S. Geological Survey Scientific Investigations Report 2009-5249, 55 p.
Izuka, S.K., 2011, Potential effects of roadside dry wells on groundwater quality on the Island of Hawai'i—Assessment using numerical groundwater models: U.S. Geological Survey Scientific Investigations Report 2011-5072, 30 p.
Figure 1. Dry wells on the Island of Hawaii