On several occasions in the last few weeks, I've walked out into the meadows around the valley and found myself puzzled by a sound rising up from near my feet. The sound is not unlike the slow bubbles of an aquarium, or the sound of the natural soda springs in the high Sierra near to where in my youth I spent many summer weeks with my family.

On closer inspection, there were bubbles all around on the soggy meadows where I was standing. These bubbles were from water sinking in, and air being displaced as the water went down into the earth. It made me realize how little I know about the mysterious goundwater system that underlies our community and so much of our history.

Of the 33 sextillion gallons of fresh water on earth at any instant, groundwater within a half mile of the surface accounts for about 11 percent. It would be safe to say that this is a lot of water. In the Western US, though, there is far less water available than might be suggested by this statistic. The seventeen western states account for some 60 per cent of the land mass of the continental US, and receive only 25 per cent of its precipitation. Most of this precipitation evaporates quickly or otherwise runs off to the sea. Groundwater that does manage to collect is the treasure of these arid regions we inhabit, a primary resource that makes life possible here.

It is hard to remember in the middle of a wet winter that rain is actually rare here. For example, from mid-February to early October last year, less than three inches of rain fell.

Fortunately for those who live here through the summer, when precipitation does come, some of it makes its way into the ground. Pulled by gravity, water seeps through cracks in rocks and through sandy or loose soils, moving down until it reaches a surface it can't penetrate, a "non-porous" material. Clay and shale are examples of such non-porous materials that form structures that water can't penetrate very well. The water then spreads out sideways, filling up an area with water until it reaches porous materials, where it continues then to flow outward and down. Eventually, all available spaces within the ground fill with water and the ground is "saturated" up to a level where it flows out through the ground into streams, springs, seeps, and other well known features that support life here in summer. The top of this "zone of saturation" usually is called the water table.

Many settlements here were situated to take advantage of groundwater features, especially springs. Some of these are still active. The building on the Plaza's southeast corner was recently retrofitted for earthquakes and during construction an old aqueduct was exposed that still flows year round with the springs that once served the Mission.

Groundwater continues to flow after the rain stops because it moves slowly through its system of cracks and loose material and has water stored uphill behind it. In some cases, the water moves downhill between layers of impermeable or non-porous rock, and builds up so much pressure that it will come out of the ground under pressure if the upper layer is broken, typically by a well. This is called an "artesian" well, named after the Artois region in France where this pressurized condition widely exists. Water also travels down through fissures to places where the earth is hot, and as happens in our area, this warm water may resurface as hot springs.

In our area, the ground is broken and reassembled many times over from California's dynamic geology, and this creates many kinds of underground structures that hold water or allow it to pass through. For this reason it is also very difficult to identify how much water exists here as groundwater, and how much we can use without affecting other parts of the groundwater system. In addition, surface water in streams are supported all summer by groundwater inflow, and in turn this water supports the great share of biodiversity in our valley. This is just one aspect of the linkage of groundwater to our surface world, and how complicated the issues are around groundwater use.

The VOM Water District is presently exploring the possibility of augmenting our local water supply with wells, and is keenly aware of this uncertainty. They are asking people in the community with wells to allow their wells to be studied so that our system of groundwater can be better understood. They are also seeking volunteers for an advisory committee to help guide this study and otherwise help explore means of supplying water here. This is an important discussion that needs careful thought and wide participation in order to assure that humans and nature can exist sustainably into the future here.

For any assumption that we are walking on "solid ground", there is water below us that reminds us otherwise, and this groundwater certainly deserves our respect and attention.