Hilton H. Cooper | USGS Science

USGS1977

Distribution of injected wastewater in the saline-lava aquifer, Wailuku-Kahului wastewater treatment facility, Kahului, Maui, Hawaii

Field studies and digital modeling of a lava rock aquifer system near Kahului, Maui, Hawaii, were employed to define and describe the distribution of planned injected wastewater from a secondary treatment facility. The lava rock aquifer contains water that is almost as saline as seawater. The saline water is below a seaward-discharging freshwater lens, and separated from it by a transition zone of varying salinity. Injection of wastewater at an average rate of 6.2 cubic feet per second is planned through wells open only to the aquifer deep within the saline water zone. The lava rock aquifer is overlain by a sequence of residual soil, clay, coral reef deposits, and marine sand that form a low-permeability caprock which semiconfines the lava rock aquifer. A two-dimensional finite-difference model was used to describe initial conditions within the freshwater part of the aquifer, and to indicate response of the freshwater part of the system to injection of wastewater. Steady-state distribution of the wastewater within the saline and the freshwater parts of the lava rock aquifer was simulated by use of a three-dimensional finite-difference model. This model, using vertical-to-horizontal ratios of hydraulic conductivity of 1:10 and 1:100, shows that wastewater with a lesser density than that of either the saline or freshwater aquifer, will form a roughly cylindrical column around the injection well base and rise through the transition zones into the freshwater lens flow system. The column diameter varies with the ratio, and the column, or plume, widens progressively through the transition zone, then maintains virtually constant size through the freshwater part of the lava rock aquifer. Seaward flow within the freshwater lens tends to distort the plume seaward. Under conditions measured and assumed for the study, it is evident that wastewater injected at the planned rate will move upward, then seaward without significant change. After reaching a new steady state, the wastewater will discharge into and through the caprock sequence within an area measuring approximately 1,000 feet inland, 1,000 feet laterally on either side of the injection site, and about 2,000 feet seaward. Little, if any, of the injected wastewater may be expected to reach the upper part of the caprock flow system landward of the treatment plant facility.

Hawaii↗

USGS1964

Sea water in coastal aquifers

Investigations in the coastal part of the Biscayne aquifer, a highly productive aquifer of limestone and sand in the Miami area, Florida, show that the salt-water front is dynamically stable as much as 8 miles seaward of the position computed according to the Ghyben-Herzberg principle. This discrepancy results, at least in part, from the fact that the salt water in the Biscayne aquifer is not static, as explanations of the dynamic balance commonly assume. Cross sections showing lines of equal fresh-water potential indicate that during periods of heavy recharge, the fresh-water head is high enough to cause the fresh water, the salt water, and the zone of diffusion between them to move seaward. When the fresh-water head is low, salt water in the lower part of the aquifer intrudes inland, but some of the diluted sea water in the zone of diffusion continues to flow seaward. Thus, salt water circulates inland from the floor of the sea through the lower part of the aquifer becoming progressively diluted with fresh water to a line along which there is no horizontal component of flow, after which it moves upward and returns to the sea. This cyclic flow is demonstrated by a flow net which is constructed by the use of horizontal gradients determined from the low-head equipotential diagram. The flow net shows that about seven-eights of the total discharge at the shoreline originates as fresh water in inland parts of the aquifer. The remaining one-eighth represents a return of sea water entering the aquifer through the floor of the sea.

Water Supply Paper↗