The Orange County Water District (OCWD) began about 10 years ago to develop an Underwater Operating Dredge to remove the clogging layer that forms on lake bottoms, preventing percolation into the large aquifer that provides the major source of well-pumped water to cover 2.3M residents in Orange County , California .
Orange County 's groundwater basin began forming millions of years ago as mountains eroded and ocean sediments filled a deep valley, trapping Santa Ana River water within the layers of accumulated sand and gravel.
Everyone knows that Southern California is historically a desert. The well-publicized battle for Colorado River water with surrounding states and Mexico dramatizes the essence of the case. Orange County 's groundwater basin provides a huge natural reservoir to provide water reliability in an area of recurring droughts and floods.
Orange County 's groundwater basin began forming millions of years ago as mountains eroded and ocean sediments filled a deep valley, trapping Santa Ana River water within the layers of accumulated sand and gravel.
The deepest aquifers of the groundwater basin still contain pristine water that fell to the earth thousands of years ago. The usable capacity is approximately 1.0 million acre-feet.
Characterization of Hardened Crust Layer in Groundwater Recharge Basins in Orange County, CA
The source water for the aquifer in Orange County is the Santa Ana River (SAR). On a seasonal basis, this river contains a broad spectrum of organic and inorganic particulate matter and dissolved solids.
The aquifer is recharged both by the SAR and a series of shallow and deep constructed recharge basins. Percolation in the deep recharge basins is impeded by the formation of a hardened crust layer (HCL) or clogging layer in the basins.
The OCWD has observed this hardened layer throughout the recharge basins on and off for the past 20 years; OCWD analyzed the HCL for general chemical concentrations, protein, chlorophyll and carbohydrate.
Scanning electron microscopy of the HCL showed that it was composed of a complex matrix of particulate matter that ranged in size from sand grains to sub-micron sized particles and a substantial number of diatom skeletons. Electron Dispersive X-Ray Spectroscopic (EDX) analysis of the HCL showed that it was composed mainly of silicates and aluminum silicates.
Silicates and aluminum silicates dominated newer HCL samples while older HCL samples contained deposits of calcium carbonate and calcium sulfate and various forms of carbohydrate. The formation of HCL is the result of a complex interaction of physical, chemical and biological processes.
Understanding the HCL on the microbial and microscopic level is essential in order to improve overall process of groundwater recharge in Orange County .
Dredging Solution
When faced with the problem of reduced percolation, OCWD began investigating vehicles that could operate underwater to remove the HCL accumulations.
Those early efforts passed through three generations of remote controlled vehicle designs intended to operate in depths of close to 100 ft. (30 m). None were particularly successful for a variety of reasons, and by 2003 OCWD issued solicitations for a floating dredge that could reach to 25 ft. (8 m) and remove the silts/clay particles and destroy the HCL.
OCWD achieved success with procuring four dredges from LWT (Liquid Waste Technology) that could operate in the shallow water recharge basins, diversions from the Santa Ana River.
Essential to the operation was the combined design and outfitting of an auger by the OCWD and LWT that can dig into the shallow sediment, separate the unwanted accumulation from the desirable sand (essential filter material). LWT's dredge achieved this objective, and operations have just begun. Improved percolation will be measured over the next six months. OCWD has positioned the four dredges into shallow-water basins alongside the Santa Ana River.
Mechanical Specifications of LWT Pit Hog™ Dredge Model BCV-4 Remotely-Controlled Dredge (BCV = "Basin Cleaning Vehicle")
| Physical |
|
| Working depth |
20' (6 m) |
| Flotation |
2 Steel Pontoons
(42"W x 46" D x 40'-0" O.A.L. x 10 gauge steel) |
| Weight (estimated) |
36,450 lbs (16,533 kg) |
Transport width w/dredging
head in place |
13'-8-1/4" (4.2 m) |
| Operational draft |
26-1/2" (673 mm) |
| Length, overall |
50'-9" (15.5 m) |
| Transport width |
11'-10" (3.4 m) |
| Height |
10'-11-1/2" (3.08 m) |
Click here for the detailed dredge specifications
About LWT
Liquid Waste Technology, LLC, originally known as Moulton Irrigation Company, was founded in the 1960s. The company expanded into liquid manure handling in 1983, and phased completely out of the irrigation business in 1984 leaving the core, Liquid Waste Technology products, focusing on environmental sludge handling equipment.
LWT concentrated on the manufacturing of the Pit Hog™ line of lagoon dredges and remote controlled lagoon pumpers, robotic submersible and track-driven pumper crawlers, rail car and tanker truck cleaning systems, digester and tank cleaning systems and custom systems for removal, agitation, transfer or land application of solids bearing liquids.
In January 2000 LWT acquired the assets of United Marine International, adding a division and new product line of marine trash skimmers and weed harvesters. In February 2003 LWT acquired the intellectual property rights and certain other assets of Innovative Material Systems, a self-propelled auger dredge product line. Both United Marine and IMS operate via independent marketing organizations.
Source: Excerpted from World Dredging Mining & Construction, January 2004
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