August 26, 2005
ON SHAKY GROUND
Collapsible soils unique in valleyCONSULTANT RECOMMENDS COUNTY DRAFT ORDINANCE TO PUT ONUS ON DEVELOPERS TO HAVE TESTS DONE ON LAND PRIOR TO BUILDING
By PHILLIP GOMEZ
Property sales disclosures would also be required, as would verifiable homeowner's insurance sufficient to cover potential damages to buildings from settling or cracking.
Hydrogeologist Tom Buqo, a veteran with more than 25 years experience in desert hydrogeology, made a presentation in July to the county commissioners regarding earth fissuring in the Pahrump Valley. Buqo recommended that developers, on obtaining a building permit for construction in areas underlain by hydro-collapsible soils, be required to submit an engineering study and undergo a field inspection for proper drainage before being permitted to begin construction. In high-risk areas, the drilling of an exploratory borehole would be required.
Buqo said there was a definite link between Pahrump Valley's ground subsidence and the withdrawal of water from its aquifer, the water table having dropped 40 to 60 feet since the 1950s. As with sinkhole problems elsewhere in the nation, the local water table's decline from area growth is the major cause of earth fissures that can destroy roads and settle houses, Buqo said.
The subsidence action so far has been limited to specific areas of the Pahrump Valley, occurring most notably with last fall's rains when a 35-foot sinkhole opened up on Hacienda Street. The street settled, in some places as much as 18 inches, and portions of pavement were washed away by surface and subsurface water flows. One home was destroyed.
In general, the area prone to subsidence ranges from north of Manse Road, between Pahrump Valley Boulevard and Homestead Road, to Calvada Boulevard, Buqo said. Other areas of subsidence have been found along Thousandaire Boulevard south of Manse.
"That part of Pahrump gets the sedimentation of Wheeler Wash and Carpenter Canyon," Buqo said. "This suggests that the geologic history of the Pahrump Valley (explains) how these fissures formed. As soon as you get a lake out in the desert, it wants to fill in with sediments."
Drainage, poor soil compaction, earth tectonics and soil types all play a role in causing earth fissuring at particular locations. Contributing human factors include past irrigation practices and water level declines.
Buqo's study was completed last spring, the valley surveyed with GPS mapping and soils field-verified as to type. A graduate student at the University of Nevada, Reno in 2004 used satellite radar imagery to take photographs of Pahrump's soils from space, identifying areas where subsidence or uplift is actively going on. The imagery provided Buqo with insights into why areas with fissures are limited to certain types of soils occurring at certain locations in the valley.
Major subsidence was pinpointed to occur along the fringes of a bowl-shaped land feature situated between the two main drainage channels coming off the Spring Mountains. Central Pahrump has been identified as the site of an ancient lake (the bowl) into which sediment was routinely deposited by mountain runoff.
Subsidence risk zones would be established in the new ordinance, according to Buqo's recommendations for county management of the problem. They would demarcate three areas:
Zone 1, comprising most of the Pahrump Valley: A subsidence study would be recommended, but not required. With a signed release, the study would be waived.
Zone 2, the "fringe" area around the subsidence "bowl" in central Pahrump. A subsidence study would be required in this largest zone and mandatory disclosures for the sale of property.
Zone 3, known areas of significant fissuring in the "bowl" itself and to the south in another smaller area. A subsidence study, proof of insurance and sales disclosure would be required.
A workshop is planned next month to inform real estate agents, developers, builders and engineers of the county's concerns in drafting the subsidence ordinance. Also in September the National Association of Engineering Geologists meets in Las Vegas and plans a field trip to Pahrump to examine its unique subsidence situation.