Am I Walking On Water?
By Andrea Peck
Groundwater has been in the news lately. The gossip on the street is that levels are low, that saltwater is intruding. There are bar graphs and pie charts. With this drought situation, we nod our heads. Yes, groundwater must be going down, there is a drought after all.
Groundwater is a term that is bandied about. We all have a sense of what it is –the word itself is self-explanatory. We know it is water that is in the ground, but what exactly does this entail?
Groundwater is important because it accounts for approximately 40% of our water supply. It serves residential, urban and agricultural needs. Groundwater fills the space between soil particles. It is filled directly when it rains or indirectly from stream runoff. Soil particles can be sand, pieces of gravel, clay, silt or the fissures and cracks between rocks often found in mountainsides. Most of our water is found where pore spaces are large and accommodating. This is usually in sandy or gravel soils.
Generally, groundwater is “held in” by hard rock surfaces such as mountain ranges. This hard surface forms a basin which is filled by naturally occurring sediment. Over time, groundwater fills the pores in the sediment.
The top zone of that, which is filled with water, is called the water table. Water that accumulates beyond the capacity of the groundwater basin discharges into streams. In this way, it completes the circle of being fed by streams and replenishing streams.
Groundwater is low enough in the earth that it is not generally depleted by surface growing plants.
Groundwater that is accessible is referred to as an aquifer. An aquifer has coarse sediment particles which make for large water-holding pore spaces. It is also accessible for a well. On the other side of the spectrum is the aquitard which has finer (often clay) sediment. The aquitard holds less water and is less permeable for well-building.
In the Central Valley alone, there are thousands upon thousands of wells. Some are small, domestic wells, while others are utilized by the agricultural community. Several thousand are used solely for public water. In a typical year, summer is the high-use season, but rainy weather during winter replenishes the supply to adequate levels.
During dry years, groundwater use can double or triple. Extreme drought can affect the aquifer in such a way that surrounding streams, once replenished by groundwater overflow, remain dry throughout the year.
But, there is hope. Water experts and researchers are looking at groundwater basins as future water storage that could potentially harbor 20 – 40 million acre feet of water. This amounts to about the same amount of surface water storage area that is currently available in California. Of course, this is a plan for “wet” years.
Ironically it's called Groundwater Banking. Perhaps, it's one we should put stock into.