On the path up to Chabot Dam, you pass this row of big tanks (click for big version).
They were used to filter the water from Lake Chabot reservoir, and they’re still filled with fine sand plus, I suppose, the decades’ worth of slime and crap they kept out of Oaklanders’ stomachs and food.
This is roughly the same purification method we rely on when we use well water. The fine pores between mineral grains purify groundwater in two ways. First, of course, they physically trap the crap. Second, the minerals themselves chemically attach to many dissolved contaminants. Clay minerals are especially important for that.
The days are long gone when Oakland was served by wells. The water table has been pulled down all over the city by human intervention. The headwaters of Lion Creek were once called Laundry Canyon because there was so much good water coming down. Fruitvale irrigated orchards for many years. The rains we get here can’t keep up, so now we pipe our water in from the Sierra.
One important feature of the new state water compact, if they can ever get it nailed down, will be a new, wide-scale program of monitoring groundwater. We need that to get a handle on the whole resource. Everyone knows, except the statutes, that surface water and groundwater are intimately connected. Western water law is an ass.