As noted in a previous blog entitled NO NEED TO DRAFT NEW DRYWELL POLICIES…ARIZONA HAS ALREADY WRITTEN THE BOOK, everything related to stormwater in the Phoenix Metro Area changed in the early 1970s, when the City of Phoenix rolled out a new stormwater retention policy that rippled across the Valley. The new policy required all new developments to retain the 100-year storm event and then dispose of that stormwater within 36 hours. Very early on, it became apparent that surface soils were getting clogged with silt and sediment and could not be relied on to drain away accumulated stormwater within the time allowed. This unforeseen problem instigated the need for an alternative method of stormwater disposal, which is when the engineered drywell emerged.
Though drywells in their simplest form have been used since the Middle Ages, this new type of drywell, developed by Torrent Resources, was fully maintainable and could last for decades. Since 1974, Torrent has been installing, testing, and improving their MaxWell®, moving through four versions of their original system as well as a dual chambered system known as a MaxWell® Plus. To date, Torrent has designed and installed more than 75,000 MaxWells® for the sole purpose of draining away retained stormwater across the Valley and elsewhere.
An unanticipated benefit of these new stormwater retention policies and use of engineered drywells, was their positive effect on the groundwater aquifers in Phoenix. In short, the stormwater policies meant to alleviate downstream flooding and erosion ended up recharging the local groundwater aquifers. And since Arizona has been infiltrating treated stormwater into the vadose zone for more than 40 years, most of our groundwater stores have increased despite the drastic increase in population over that same time period.
To understand and quantify this benefit, the City of Chandler tasked GeoSystems Analysis (GSA) with studying the effects of urbanization on their groundwater resources. The 2004 study showed that urbanization (which typically increases runoff and reduces infiltration), stormwater retention, and drywells play a significant role in increasing the volume of water recharged to the aquifers that lie beneath Chandler. GSA estimated that groundwater recharge rates soared from 191 ac-ft/yr on undeveloped land to between 2,100 – 3,100 ac-ft/yr after build out…an increase of 1,000% – 1,500%! The study also defined the dry year and wet year recharge rates for fully developed land at 770 ac-ft/yr and 8,700 ac-ft/yr, respectively. The study concluded that urbanization and stormwater retention in Chandler had a very beneficial effect on their aquifer volumes and created a significant water resource for the City’s future.
With the current drought across much of the western U.S., some might consider using stormwater to recharge our groundwater aquifers to be more important than preventing flooding and erosion…well, maybe they’d think it at least equally important.
The GeoSystems Analysis Study can be found here: GeoSystems Analysis – Chandler Study
Drought Monitor: Southwest Drought