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