GROUNDWATER REPLENISHMENT

Spreading grounds

Rain water

Stream

Lake

State water project

Since the most abundant source of water for the San Gabriel Valley is groundwater pumping, water suppliers are focused on old and new means of groundwater replenishment. Groundwater levels and supplies, much like surface water levels and supplies, fluctuate in cycles based on climate and usage levels. More groundwater is pumped out than is naturally replenished. Contamination also can reduce the quantity of our underground water supply.

As the drought in Southern California has worsened, drying hillsides, surface streams, rivers and lakes are visible reminders of water shortages. It's important to extend our thinking to realize that underground sources of water ebb and flow also based on similar factors. There are a variety of measures of underground water supply or storage levels and most of these in the San Gabriel Valley indicate our groundwater supply system is both stressed and reaching historically low levels.

The natural cycle or process of water reaching underground aquifers and wells takes time and is affected by geologic factors and sub-surface moisture levels. The process of water seeping, filtering or percolating down to where it may be safely pumped for use can take many months, so replenishment of groundwater supplies does not happen overnight.

The various ways in which groundwater may be replenished include:

• rainwater occurring naturally over local and mountain areas
• water filtering down to groundwater from lakes, streams and rivers
• water flowing from mountain canyons and streams that is controlled and managed by systems of dams, reservoirs and spreading grounds
• underground water storage systems
• imported water that is transported by pipelines and aqueducts into spreading grounds
• treated waste water that is transported by pipeline to spreading grounds and/or used for irrigation and agricultural purposes (e.g. Orange County Groundwater Replenishment System, G.R.I.P.)

Spreading grounds provide a place for surface water to recharge underground aquifers. They can be constructed to collect rain water, water flowing from mountain canyons and streams, imported water, and purified wastewater pumped from water treatment facilities.

Spreading grounds are an important water supply feature in the San Gabriel Valley as we have many areas geologically suited to the way they work. Spreading grounds must be constructed in geologically suitable areas where surface water can soak down into the subsurface and recharge the aquifers in a groundwater basin. For a spreading grounds project to work, the soil beneath it to the water table must be highly permeable sand or gravel so the infiltrating surface water can move downward without blockage. If less permeable silt or clay layers exist, they can act as a barrier and prevent the water from reaching the water table and aquifers.