Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast Lake Counties, Florida

By Andrew M. O'Reilly
U.S. Geological Survey, 224 W. Central Parkway, Altamonte Springs, Florida 32714

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Abstract

Wastewater reclamation and reuse has become increasingly popular as water agencies search for alternative water supply and wastewater disposal options. Several governmental agencies in central Florida currently use the land-based application of reclaimed water (wastewater that has been treated beyond secondary treatment) as a management alternative to surface-water disposal of wastewater. Water Conserv II, a water reuse project developed jointly by Orange County and the City of Orlando, began operation in December 1986. In 1995, the Water Conserv II facility distributed approximately 28 Mgal/d of reclaimed water for discharge to rapid infiltration basins (RIBs) and for use as agricultural irrigation. The Reedy Creek Improvement District (RCID) began operation of RIBs in September 1990, and in 1995 these RIBs received approximately 6.7 Mgal/d of reclaimed water. Analyses of existing data and data collected during the course of this study were combined with ground-water flow modeling and particle-tracking analyses to develop a process-oriented evaluation of the regional effects of reclaimed water applied by Water Conserv II and the RCID RIBs on the hydrology of west Orange and southeast Lake Counties.

The ground-water flow system beneath the study area is a multiaquifer system that consists of a thick sequence of highly permeable carbonate rocks overlain by unconsolidated sediments. The hydrogeologic units are the unconfined surficial aquifer system, the intermediate confining unit, and the confined Floridan aquifer system, which consists of two major permeable zones, the Upper and Lower Floridan aquifers, separated by the less permeable middle semiconfining unit. Flow in the surficial aquifer system is dominated regionally by diffuse downward leakage to the Floridan aquifer system and is affected locally by lateral flow systems produced by streams, lakes, and spatial variations in recharge. Ground water generally flows laterally through the Upper Floridan aquifer to the north and east. Many of the lakes in the study area have no stream inflow or outflow, because the mantled karst environment precludes a well developed network of surface-water drainage.

The USGS three-dimensional ground-water flow model MODFLOW was used to simulate ground-water flow in the surficial and Floridan aquifer systems. A steady-state calibration to average 1995 conditions was performed by using a parameter estimation program to vary values of surficial aquifer system hydraulic conductivity, intermediate confining unit leakance, and Upper Floridan aquifer transmissivity. The calibrated model generally produced simulated water levels in close agreement with measured water levels and was used to simulate the hydrologic effects of reclaimed water application under 1995 and proposed future conditions.

Based on historical data, increases of up to about 40 ft in the water table and less than 5 ft in the Upper Floridan aquifer potentiometric surface had occurred by 1995 as a result of reclaimed water application. The largest increases were under RIB sites. Model results generally agreed with these data. An average traveltime of 10 years at Water Conserv II and 7 years at the RCID RIBs was required for reclaimed water to move from the water table to the top of the Upper Floridan aquifer. Elevated chloride concentrations measured in Whittenhorse Creek and Perimeter Canal indicate that reclaimed water from the RCID RIB site probably is discharging to adjacent surface-water features. Approximately 67 percent of the reclaimed water applied at the RCID RIB site recharged the Floridan aquifer system, whereas 33 percent discharged from the surficial aquifer system to surface-water features; 99 percent of the reclaimed water applied at Water Conserv II recharged the Floridan aquifer system, whereas only 1 percent discharged from the surficial aquifer system to surface-water features. At Water Conserv II, an unquantified amount of reclaimed water is lost to evapotranspiration at agricultural irrigation sites. The majority of reclaimed water applied at both facilities probably ultimately will discharge from the Floridan aquifer system outside the model boundaries.

Proposed future conditions were assumed to consist of an additional 5.9 Mgal/d of reclaimed water distributed by Water Conserv II to discharge to new RIBs and to irrigate the new Orange County National Golf Center, with an additional 5.8 Mgal/d of reclaimed water discharged to RCID RIBs. Increases of up to about 20 ft in the water table and 2 ft in the potentiometric surface of the Upper Floridan aquifer were simulated. The directions of reclaimed water movement through the ground-water system generally were similar to those under 1995 conditions. However, the greater reclaimed water application rate at the RCID RIBs caused approximately half of the RCID reclaimed water to discharge to surface-water features and half to recharge the Floridan aquifer system.

In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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