Potential Effects of Roadside Dry Wells on Groundwater Quality, Island of
Hawai‘iAssessment Using Numerical Groundwater
Models
A study in cooperation with the Hawaii County Department of Public Works
Scot Izuka, USGSPresented at the Big Island Water Resources MeetingMarch 21, 2011
Dry Wells
Minimum capacity ~5 ft3/s
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
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
Conceptual Model
Infiltration = 5 ft3/s for 1 hour Conservative contaminant (no decay, sorption, chemical reaction); concentration = 100
Thin UZ -- higher conc. Thick UZ -- lower conc.
Tested Effect of Unsaturated-Zone Thickness
Concentrations diminish quickly as plume moves downgradient
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 well700 ft from dry well, concentrations < 1% of concentration in infiltration water0.5 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
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
Use of Results
Hypothetical dry well in Kona, 500 ft from receiving water,50-ft above water table
Estimated concentration at receiving water
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