According to USGS, Maunawili and all of its streams sit upon the Ko'olau Basalt Aquifer (Figure 55). The Ko'olau Basalt is the principal aquifer of Oahu; sedimentary deposits are poorly permeable and yield little water. These deposits form a caprock that confines water in the Ko'olau Basalt inland and in the coastal plains. Regional ground-water movement is from the highlands to adjacent ground-water areas and directly to the ocean. Dike-impounded water is most important in this ground-water area, and some water levels are as much as 1,000 feet above sea level. Mean annual predevelopment recharge was about 368 million gallons per day and was entirely from rainfall. Discharge is to streams and by ground-water outflow to adjacent ground-water areas; withdrawals from wells, shafts, and springs; evapotranspiration; and outflow to the ocean.

USGS LINK

https://pubs.usgs.gov/ha/ha730/ch_n/N-HItext3.html#:~:text=The%20north%2Dcentral%20Oahu%20ground%2Dwater%20area%20composes%20the%20northwestern,confining%20unit%20of%20sedimentary%20deposits.