Groundwater from Construction Sites, along with Recycled Water and Storage Tanks, could make a Significant Contribution to Palo Alto Water Conservation Goals
By: Joseph Rahn - The Purple Pipe Company
As we head into 2018, the Palo Alto Public Works department is continuing to draft regulations associated with the way groundwater resources must be addressed when digging a basement at a residential property. To construct a basement, any groundwater encountered must be pumped out before concrete can be poured. The homeowner/builder has two options to limit the amount of groundwater pumped: A.) Install “secant walls” at the property to cut off the lateral flow of groundwater, or B.) Have a geo-technical report commissioned and hire a water truck company to recapture and reuse the water on city landscaping (trees, pocket parks, median strips, islands, etc.) during one day per week of de-watering.
A.) Secant Walls
The secant wall approach to basement construction can almost entirely prevent groundwater from going down the city storm-drain. Secant walls are an arrangement of concrete-filled steel piles which stop the lateral flow of groundwater. This limits the amount of water needed to de-water to what water is in, or is coming up from underneath the walls. However, unless there is other groundwater available, the walls have to be made with potable drinking water because recycled water cannot be used (the construction slurry doesn't bond properly). Secant walls are generally implemented on larger scale commercial building projects. The biggest concern with secant walls, is that they’re permanent and will most likely disrupt the f low of underground aquifers/rivers. A basement generally goes down about 15’ below the surface, but secant walls go down to about 30’ - 35’ and are permanent. This could disrupt the flow of groundwater in the vicinity. This would ‘reroute’ the amount of water flow to adjacent properties, and can affect the water accessed by trees.
Secant walls are expensive and add appreciably to the cost of construction. If there is a change of building plans for the property, secant walls, unless taken out, may limit what a homeowner or architect could do. Would the difference in these costs to implement secant walls be better allocated to a more holistic water conservation project such as recapture and reuse?
B.) Recapture and Reuse
With the current City of Palo Alto water truck recapture/reuse program, only a small fraction of what is travelling down the storm-drain gets recovered. When a water truck comes to pick-up groundwater, it takes about 10 minutes to fill a 2,000 gallon truck. The water is then used to irrigate landscaping within the City of Palo Alto.
It typically takes 1 or 2 hours to apply a tank full of water. Many of the sites irrigated are small islands or median strips, and it’s not often that a truck could use an entire tank of water at any one location. Delivery and getting into and out of watering positions takes time. So during an 8 hour day, an average of 4 or 5 tanks or so is available for city watering.
If there were water storage tanks, or cisterns, installed at city parks, a water trucking company could dramatically cut down delivery time. These tanks could be buried underground and have various capacities, depending on site conditions. In general, they might be slightly larger than the amount of gallons needed to replace one park irrigation cycle, which on average, might be about 20,000 gallons. With the addition of a truck fill pipe, the water could simply be dumped into the tanks and stored until the next irrigation cycle. This would speed up the delivery process and allow recovery of up to 20% of the groundwater pumped at high-flow sites (depending on how much innovation takes place). Assume one water truck was assigned 5 days a week to a dewatering site, pumping 800,000 gallons of water per week. The weekly 100,000+ gallons recaptured per truck could be used to replace the potable drinking water used for landscaping at Palo Alto city parks or sports fields. Not only would this “double save” water, but it might be enough to incentivize adding on additional investment in rain-catchment features.
Potential sites for water tanks/cisterns would be city parks or sport fields near the dewatering zone. Parks and fields which are unlikely to be connected any time soon to the planned regional recycled water pipeline extensions would make the best candidates to be served by the groundwater program. The water tank and pumping system, along with the park’s sprinkler system, would be retrofitted and put on a closed loop system. A true air-gap would allow backup municipal water to be routed into the tank. Therefore, while the system could use municipal water, it normally would function with reclaimed groundwater or recycled water. Depending on the site, some systems could also be outfitted with rain-harvesting features to double as rain-catchment during the rainy season. Please see the illustration at the end of the article for possible tank locations. Installation of tanks would not be without its challenges. There would need to be parking or access to a fill port and a location for each cistern. Groundwater levels may be a limiting factor on the depth of excavation, although shapes and sizes of tanks can be customized.
Due to the logistics of delivering water and the demand for it needed at each park, recycled water could be delivered into the tanks year round (assuming Title-22 approval). Most likely, there would be extra hours from the groundwater season (April - October) in the city’s watering account. So when the groundwater season is done, these extra hours could be applied to delivering recycled water.
Once a few tanks are installed, it should be possible to turn a truck load around every 30 - 45 minutes (depending on access and distance to cistern) and deliver 10+ full truck loads per day (20,000+g) per truck. The trucks would also be completely empty each time they were filled at a groundwater site (logistics wise). If the homebuilder paid a fee for 5 days per week, and there were enough tank capacity in place, that would be 100,000+ gallons per week saved. In an average 2.5 month dewatering period, this might recapture 10 - 15% of the 15-16 million gallons of water. Innovations could be made to truck filling, reducing it to less than 5 minutes and so that it doesn’t block the sidewalk. Recapture could exceed 200,000 gallons per week with the right equipment (pumps and fill pipes) in place. The costs to the homebuilder over the 2.5 month period would be less than $60,000 which is most likely below the extra costs associated with building secant walls. Any extra money could be put into a fund to build out park/sports field infrastructure.
Trucking water does burn diesel fuel and that is a downside for the environment. There are always trade-offs to consider. However, driving distances would be kept very short because the tanks would primarily be installed in the de-watering/groundwater zones. In the future, electric service vehicles and even automated electric trucks could be utilized. The pumping at groundwater sites and dumping into tanks could be done with electric pumps as well. So it’s possible that the overall operation could be very quiet, opening up the potential for nighttime/early morning delivery. Lastly, the usage rates of the various types of water used at city parks and sports fields could be monitored remotely online over the cloud.
On its own, trucking water may not always make sense relative to the secant wall method. However, there are many trade-offs: costs, permanent groundwater diversion, length of construction projects, etc. If trucking were combined as an integrated/holistic approach to saving water... it will make sense. If the daily average irrigation cycle at a city park or sports field is 20,000 gallons, essentially the majority of potable water use at targeted sites could be replaced, year round. Rainwater harvesting by itself may not always be cost-effective, as the tanks are often dry during the dry season when the landscape needs it most. However, with the addition of a truck fill pipe for recycled water, installing rain-harvesting water storage tanks could make sense. If groundwater were used with, or instead of recycled water, than the city would benefit from installing the tanks/retrofitting the systems with the money that the homebuilder would set aside. The homebuilder would probably like the fact that they would be helping the city replace large irrigation cycles at parks.
Although the recapture and reuse method overall doesn't stop as much water down the storm-drain the way a secant wall could, the holistic approach to replacing potable water use at city parks could offset the concerns of the diversion of groundwater down the storm-drain. It probably depends on how one weights the value of each type of water. With the ever greater demand on California’s water resources, and fluctuating amount of seasonal rainfall, a holistic approach seems to be a sound policy. Groundwater from construction sites, along with recycled water and storage tanks, can make a significant contribution to Palo Alto’s water conservation goals.