Tucson and Phoenix have rather different views of something called a “drywell”. In Phoenix there are about 100,000 drywells. In Tucson there are about 0.
Drywells are exactly that – dry wells. You put water into them, rather than take it out. After filtering thru hundreds of feet of dirt, a network of drywells could potentially add 3 billion gallons (or more) of new water to our groundwater supply in an average year.
I first discovered this potential when I ran across a report on the Chandler drywell system prepared by GeoSystems Analysis. (Their Tucson office is on North Dodge.) According to hydrology and soils engineer, Mike Milczarek, all the cities in the Phoenix area use drywells extensively, because developments are required to keep most stormwater on their properties. Drywells and landscaped detention basins are their method of choice.
About three decades ago, Pima County and Maricopa County parted ways on the issue of groundwater pollution. This was a time when Tucson was very concerned about TCE pollution from aerospace and railroad operations in the southwest part of the City. Since then, the Arizona Dept of Environmental Quality has established rules for drywells and permits them for stormwater runoff if they are at least 20 feet above the aquifer. In most part of town, Tucson’s aquifers are over 300 feet below the surface.
The Chandler study found that an average of over 4000 acre-feet of rainfall was being recharged by nearly 4000 drywells, though this could be as little 800 AF in a really dry year, and over 10,000 AF in a wet year. A little less than 25% of the runoff from nearby areas actually ends up as recharge. The rest is lost because of a poorly understood fact – the larger the watershed, the greater the fraction that evaporates before it gets somewhere.
According to Milczarek, an extensive network of drywells is quite feasible for metro-Tucson (which has 6 times the area as Chandler, and gets nearly 50% more rain). The key requirement to make it happen here will be to find influential champions who will actively support the effort.
So what could we do with billions of gallons of recharge in a network of drywell and the billions more gallons that at least make it to the edge of the street?
We could convert Tucson into the Land of 10,000 Micro-Parks. And create up to 100 square miles of edible urban forest throughout Tucson. We could use these parks and urban food forest to calm the traffic in our neighborhoods and turn them into pedestrian-safe and shaded parks in the form of an Edible Oasis in the Sonoran Desert.
It’s time to get moving, Tucson. Phoenix is way ahead of us! And we are wasting billions of gallons of renewable water each year.
(The full report on Chandler’s Drywell system can be found here.)