County looking to convert two West Maui wells

Proposal would turn exploratory wells into full production wells

Maui County wants to turn a pair of exploratory wells — originally drilled with the goal of finding new groundwater sources — into permanent production wells to service West Maui customers.

The West Maui Water Source Development Project would convert the Mahinahina Well and the Kahana Well into production wells. It would also involve the construction of a 500,000-gallon control tank, transmission waterlines and access roads for both well sites.

A draft environmental assessment for the project was published in the April 8 edition of the Office of Environmental Quality Control’s “The Environmental Notice.”

The county needs additional water sources to meet the demands of the growing West Maui population, and also to make up for the water that it must return to Kanaha Stream.

In November, the state Commission on Water Resource Management set in-stream flow standards for Kahoma Stream and its smaller tributary, Kanaha Stream, both of which were diverted in the late 1800s to irrigate sugar cane fields on the west side.

The county Department of Water Supply has been managing the Kanaha Stream diversion at the 1,050-foot elevation level since Pioneer Mill shut down in 1999. With the ruling, the department was required to release 550,000 gallons a day past its diversion, with additions of 130,000 and 120,000 gallons per day to meet the needs of loi kalo planted in the area.

The two exploratory wells, Mahinahina and Kahana, have been drilled in recent years as potential backup sources of potable water. As groundwater sources, they could also provide more reliability to the West Maui system.

The availability of surface water is reduced during periods of drought, severe rainfall and maintenance to the ditches that service the Mahinahina Surface Water Treatment Plant, for example. In these instances, the department relies on groundwater sources.

Future water demand in the West Maui area is expected to increase from 9 million gallons per day to 16 mgd by 2035, according to the Maui Island Water Use and Development Plan, which the department is currently working to update.

The Mahinahina and Kahana wells would provide 1.63 mgd of additional water to meet projected needs, while also serving as a backup for existing users.

Under the county’s proposed plans, the Mahinahina Well site would include a well pump producing 700 gallons per minute, a 300,000-gallon control tank and a 52-foot-long, 20-foot-wide concrete masonry unit control building to house the chlorination system, supervisory control and data acquisition system and electric equipment. The well would draw 0.672 mgd from the Honokowai Aquifer, based on an operational schedule of 16 hours of pumping per 24-hour period.

The Kahana Well site would include a 1,000-gallon-per-minute well pump, a 30,000-gallon control tank and a 56-foot-long, 20-foot-wide concrete masonry unit control building to house the same equipment as the Mahinahina site. The well would draw 0.96 mgd from the Honolua Aquifer.

The two wells are located about 3 miles mauka of Honokowai Beach Park. The Mahinahina Well is on state lands; the Kahana Well sits on property owned by Maui Land & Pineapple Co.

The project also calls for installing a 500,000-gallon control tank, which will be located on a 1.8-acre site next to the Mahinahina treatment plant.

Water from both wells would be pumped into control tanks on each site. The water would go through a chlorination unit for treatment and be transported by gravity to the 500,000-gallon control tank. From there, the water would be transmitted to the department’s existing 2 million-gallon Honokowai storage tank located about 1 mile west of the Mahinahina treatment plant.

Overall, the project would cost an estimated $18.4 million and would likely be carried out in two phases due to funding constraints. Once the county goes through the environmental assessment process and gets the necessary permits, the Kahana Well and the necessary waterlines and access roads would go out to bid. Construction would take about 18 months from the notice-to-proceed date. The Mahinahina Well site would be developed at a later date.

While the project area covers several large parcels of land spanning 3,501 acres, the improvements would be limited to 11.5 acres.

Comments on the project are due May 8. They can be sent to Curtis Eaton at the county Department of Water Supply, either by phone at 270-7835, by email at dws.engineering@mauicounty.gov or by mail at 200 S. High St., fifth floor, Wailuku 96793.

To view the full draft assessment, visit oeqc2.doh.hawaii.gov/EA_EIS_Library/2019-04-08-MA-DEA-West-Maui-Source-Development-Project.pdf.

* Colleen Uechi can be reached at cuechi@mauinews.com.