MAPPING AND MONITORING EARTH FISSURES IN ARIZONA: A WIDESPREAD MAN-MADE GEOHAZARD

Arizona has the dubious honor of having more earth fissures than any other state in the nation. Earth fissures are tension cracks in basin sediments that form due to differential land subsidence. Fissures have formed in deep basins throughout central and southern Arizona within and along the margins of subsidence zones due to excessive groundwater withdrawal. Overdrafting began in the 1930s to support an expanding agricultural footprint. Groundwater level declines in excess of 500 ft and associated ground subsidence of up to 20 ft is recorded in some basins. Widespread fissure formation began in the 1960s and continues today. The Arizona Geological Survey has mapped and monitors over 185 miles of earth fissures in 27 study areas throughout the state.

Fissures are an insidious hazard as they can go unnoticed at the surface until a deep chasm hundreds to thousands of feet long appears. Fissures initially manifest as narrow hairline cracks but are gradually enlarged by subsurface erosion which can lead to abrupt collapse of overlying sediment. The sudden appearance of an earth fissure often occurs following intense rains where surface flow is intercepted by a fissure, resulting in rapid erosion and collapse. Fresh fissures are characterized by sharp, abrupt edges, unstable near-vertical walls, a tapered profile (width decreases with depth), and an open narrow crack which extends to unknown depths. Hazards of earth fissures include damage to homes, infrastructure, property, transmission of surface contaminants deep underground, and redirection of floodwaters.

The introduction of Colorado River water for agriculture and groundwater recharge by the Central Arizona Project (CAP) Canal in the 1980s brought about a period of groundwater recovery and waning land subsidence yet the damage and effects from fissures formed in the past persist today. Basins in SE Arizona with large agricultural footprints lacking access to external recharge are experiencing rapid groundwater lowering and coincident land subsidence and fissure formation annually. Ongoing drought has led to cutbacks on available CAP water which means farms in recovering basins will be switching back to groundwater. Land subsidence and earth fissures are clear symptoms of our overuse of groundwater reserves yet are unaddressed by policy.