Subsidence -

When the Ground Sinks Beneath Us

Published in the EarthSphere Blog: Feature Image: That Sinking Feeling (©Archean Enterprises, LLC; ArcheanArt)

Terra firma is not as solid and unyielding as we may believe. Sometimes it fades away before we even notice it’s gone. The headlines of a recent article in the San Francisco Chronicle lament the slow descent of some California towns, where ground levels have dropped nearly a foot in one year. Seemingly firm solid ground is sinking along with the homes, roads, and farms it supports.

The problem is prominent in the San Joaquin Valley, and it’s not a new or mysterious phenomenon. During the 50-year span between 1920 and 1970, parts of the valley subsided by as much as 28 feet. While climate change has exacerbated the problem, it is not the root cause.

As is often the case, the residents and businesses of the valley are the architects of their own demise. Agriculture is big business in the valley, and the sine qua non of profitable farming is plenty of water.

Water supply is very simple to understand. It will arrive by falling from the sky, flowing through rivers and streams, or by being pumped from the ground. The choices are limited, but the interaction between these variables is complex both scientifically and legally.

The sinking feeling for farmers in the San Joaquin Valley results from the interplay between surface water and groundwater. When surface water is scarce, farmers resort to pumping water from the ground. To the uninitiated, this seems almost miraculous — water continuously flowing from Mother Earth, keeping the soil moist and the crops healthy. But there is no free lunch.

Groundwater

Groundwater, like all other natural resources, is subject to the laws of physics. Water takes up space, and only so much of it can exist in a finite space. Fortunately, when we pump water from the ground, rainfall and surface water recharge the subsurface aquifers and replenish our supply.

The balanced equation of water-in and water-out works until the point where we pump more groundwater than the surface supplies can replenish. This is the part where climate change comes into play. In a dryer and hotter world, there is less surface water. But farmers depend on their businesses to feed families and pay employees, so they understandably keep pumping. The dryer the surface conditions, the more they pump. As you contemplate the pluses and minuses of this scenario, I am sure you sense impending trouble. This trouble comes in two forms: wells dry up, and the ground deflates in a process called land subsidence.

Dry water wells are usually the first indicator of over-pumping. But below the ground, other changes take place. Because the aquifer is primarily composed of water and porous rock, water residing in the rock and soil beneath our feet takes up space and provides some structural support. Pressure from the water pushes against grains of rock and clay, keeping the pore spaces open. This structural support is lost when water is removed, and rock grains collapse into the empty pore spaces. The space a cube of land takes up is a combination of rock, soil, and water. When the water disappears, the land volume decreases. It accomplishes this by subsiding.

If enough water is removed, then the aquifer may collapse. The collapse process packs the sand and silt grains closer together, thus leaving less pore space for new water to fill. This process is irreversible, so even if the aquifer fills up again, it can never hold as much water as it initially did.

Aquifer Collapse

The effects of aquifer collapse extend to the surface in some areas resulting in sinking ground levels. In Cochise County, Arizona, land levels dropped by 10 feet due to the over-pumping of the local aquifers. Stresses related to the collapse created cracks and fissures in the ground. About 42 miles of fissures have been documented. The surface trenches from these fissures are up to 10 feet wide and 30 feet deep. The shifting ground also breaks waterlines and destroys building foundations.

Land subsidence from overpumping is serious business. Indonesia’s current capital, Jakarta, is slipping below sea level and is beset by a host of environmental problems, including land subsidence. The government announced its long-range plan in 2019 for dealing with Jarkata’s issues — migrate and move to higher ground. Plans call for the government to move its operations to a new capital city named Nusantara. This migration could start as early as 2024.

Jakarta is one of the fastest sinking cities on Earth. It is home to 33 million people, and the city has the unwanted distinction of suffering from two anthropogenic effects — the ocean is rising from climate change, and the land is falling from overpumping.

The problem of overpumping is global, driven by various combinations of ignorance and greed. It is unfair to simply blame subsidence in the San Joaquin Valley on the farmers. The root of the problem is the failure of the State to do its job and regulate water usage in a way that provides for a sustainable future for everyone. The best time for establishing that framework was about 100 years ago, well before we reached a crisis point. The second best time is now. But it won’t happen without pain, and with distinct winners and losers.