Overcoming the Black Plug Layer Problem: A Case Study at SRP's NAUSP Facility

As the need for sustainable water usage grows, understanding soil health becomes increasingly important. A key issue affecting soil health and water absorption is the formation of a "black plug layer". This troublesome layer, essentially an anaerobic slime, can cap percolation ponds, inhibiting the absorption of water into the ground. It's a common problem faced by water banking operations, and one that was recently addressed at the Salt River Project's (SRP) New Arizona Ultimate Storage Project (NAUSP) facility in West Phoenix, Arizona.

SRP, the third-largest private water utility in the United States, operates the NAUSP facility. This significant establishment serves as a water banking site, a critical element in conservation efforts in the arid Southwest. The facility mixes reclaimed water from the surrounding four municipalities with Colorado River water, delivered via the Central Arizona Project canal system. The mixed water then slowly percolates into the groundwater table through six 20 to 25-acre percolation basins, essentially storing it for future retrieval and use.

The NAUSP facility has been notably successful, banking nearly one million acre-feet of water into the ground over the past five years. However, the formation of the black plug layer posed a severe issue that brought water banking operations to a standstill.

Traditionally, the solution to this problem was tilling the earth using heavy machinery to reopen the soil and continue operations. In the case of the NAUSP facility, this meant deploying a bulldozer equipped with a seven-foot-long steel tooth across all basins. However, one basin was set aside for a different approach. SRP sought a more cost-effective and equally efficient solution than the $500,000 spent on tilling, and they turned their attention to CBX.

CBX is a product known for its beneficial effects on soil health, containing numerous aerobic strains of active biologicals. Given its relatively low cost and the ease of application, CBX appeared as a valuable alternative to traditional tilling methods.

On March 31, CBX was applied to the sixth basin at a rate of two gallons per acre. Water was subsequently introduced, and over the next two weeks, the basin filled up. Interestingly, before the CBX application, it took only three days to fill the basin. However, with CBX, it required two weeks, indicating a significant increase in the soil's water-holding capacity. Once the basin was filled, the gates were closed, and remarkably, the water in the basin drained within 48 hours.

This case study from SRP's NAUSP facility provides an intriguing insight into managing the black plug layer problem. By using CBX, the facility was able to enhance water absorption, thereby maximizing their water banking operations. It's a promising solution for similar facilities grappling with the black plug layer issue, underlining the importance of understanding and improving soil health for sustainable water use.