The basal water quality of the Hawaiian Commercial and Sugar Company (HCandS) aquifer is most affected regionally, as well as locally, by the prevailing agricultural practices. The deterioration of the water is due in part to fertilization and to a greater extent to heavy pumping and recycling of the basal water. Water quality in the Pioneer Mill area parallels that of HC and S, although on a regional basis, the basal water quality, unlike that of the Pioneer Mill area, is not as deteriorated. Local effects of pumping are also especially noticeable in the Pioneer Mill area. Ground-water quality in the Kahuku area shows the obvious presence of irrigation return water indices, but, unlike the two plantations on Maui, the magnitude of the increases relative to uncontaminated water sources is considerably smaller. The effect of fertilization on Kahuku may be considered to be a principal factor in the regional distribution of index constituents with a relatively uniform nitrate distribution throughout. Local effects of pumping are quite pronounced and influence overall increases of indices, indicating that where heavy pumping takes place for irrigation, the increase in index constituents are correspondingly greater. (Author)