1. Potential Effects of Roadside Dry Wells on Groundwater Quality, Island of Hawai‘i Assessment Using Numerical Groundwater Models A study in cooperation with the Hawaii County Department of Public Works Potential Effects of Roadside Dry Wells on Groundwater Quality, Island of Hawai‘i Assessment Using Numerical Groundwater Models A study in cooperation with the Hawaii County Department of Public Works Scot Izuka, USGS Presented at the Big Island Water Resources Meeting March 21, 2011

2. Dry Wells Minimum capacity ~5 ft 3 /s

3. Problem More than 2,000 dry wells on Big Island dispose storm- water runoff Concern that these dry wells could provide a contamination route to receiving waters

4. Objective and Scope Assess the potential for dry wells to affect quality of receiving waters Results should have island-wide applicability Approach Numerical groundwater flow and solute-transport modeling Multiple simplified models simulating various conditions

5. Conceptual Model Infiltration = 5 ft 3 /s for 1 hour Conservative contaminant (no decay, sorption, chemical reaction); concentration = 100

6. Thin UZ -- higher conc. Thick UZ -- lower conc. Tested Effect of Unsaturated-Zone Thickness Concentrations diminish quickly as plume moves downgradient

7. Tested Difference between Hilo and Kona For comparable unsaturated-zone thickness, Hilo had lower concentrations than Kona In both: Concentrations decline quickly as plume migrates away from dry well 700 ft from dry well, concentrations < 1% of concentration in infiltration water 0.5 mi from dry well, concentrations ≤ 0.1% of concentration in infiltration water Hilo model had less permeable aquifer, higher groundwater flux (wetter climate) than Kona

8. Infiltration Focused in Dry Well vs. Spread “Naturally” over Large Area Directly beneath infiltration area, dry-well case yielded much higher concentrations than “natural” case Few hundred feet away, difference is small 0 200 400 600 800 1,000 Dry well - infiltration focused in small area Natural – infiltration over large area

9. Use of Results Hypothetical dry well in Kona, 500 ft from receiving water, 50-ft above water table Estimated concentration at receiving water

10. Infiltration focused at a dry well can cause contaminant concentrations in groundwater to be substantially higher near dry well Concentrations decline quickly downgradient from well Thicker unsaturated zones result in lower concentrations Differences in climate and aquifer properties result in lower overall concentrations in Hilo compared to Kona Results can be used to approximate dry-well effects in other areas of Big Island Summary