09-26-2010, 06:29 AM
My parents live in central NY at an elevation of about 850 feet. They are now on county water but for more than 25 years they relied on a well. The well was only 85 feet deep. Their neighbor 2 lots down had to drill more than 400 ft to get "good" water but had hit water at only 100' or so. This is normal in many parts of the world where the water table (the line where the water stops trickling directly down and starts spreading out sideways) follows the contour of the land. This is due to millions of years of weathering and sedimentation during which impermeable layers develop. Hawaii in general and the big island in particular is pretty young for these processes to have occurred and in general all the islands are giant piles of fractured rubble with intermittent impermeable layers of pahoehoe. While I know that the board of water supply for Honolulu does find pockets of water well above sea level called perched water, they have huge advantages as far as economy of scale goes to find and develop these sources. The rest of us must count on drilling down almost to sea level which is where both rain and sewage goes. Depending on how porous the rock is and how high above sea level you are it can take decades for the water to trickle down, after which you could probably drink the sewage without harm. If however you are near sea level and the rock is porous it can get there very quickly. This definitely can and does happen here in Hawaii as it does elsewhere and it means that you can be swimming in tainted water when you go to the beach or even a well can be tainted. The thing about Hawaii is that the ground is generally so porous. Every slab of pahoehoe has an edge or crack in it somewhere so pretty soon the water goes straight down.
So how is a cesspool different than a septic system? A cesspool is a deep hole in the ground that makes no effort to spread the sewage out over a wide area. The design inherently speeds the sewage on its way down. A septic system is a very shallow hole in the ground that is required by code to spread the sewage out over an area proportional to the expected flow and to deliver the sewage to the top couple of feet of soil where there is oxygen present and where there is tremendous surface area in the surrounding sand and gravel backfill on which aerobic bacteria can live and consume the pollutants. The septic tank also has features to capture solids before the liquid gets to the leach field, in contrast to a cesspool where the solids are free to migrate into the soil. This is not so much to protect the environment. Whether it is water or solids, if it has essence of poo in it it is equally bad and in fact the water is more mobile. The reason the septic tank captures the solids is that the leach field is intentionally designed with very small gravel to get very high surface area. For purely mechanical reasons the solids, if they get through, will clog up the leach field. That is the reason that septic tanks must be pumped regularly. Cesspools sometimes also need to be pumped but by the time that happens you are screwed because it means that the little cracks are all full of solids and the water can't escape. If you let the septic tank go that far you would be equally screwed, but at least with the septic tank, periodic preventive pumping can maintain the porosity of the leach field indefinitely by preventing solids from ever getting to the leach field.
So how is a cesspool different than a septic system? A cesspool is a deep hole in the ground that makes no effort to spread the sewage out over a wide area. The design inherently speeds the sewage on its way down. A septic system is a very shallow hole in the ground that is required by code to spread the sewage out over an area proportional to the expected flow and to deliver the sewage to the top couple of feet of soil where there is oxygen present and where there is tremendous surface area in the surrounding sand and gravel backfill on which aerobic bacteria can live and consume the pollutants. The septic tank also has features to capture solids before the liquid gets to the leach field, in contrast to a cesspool where the solids are free to migrate into the soil. This is not so much to protect the environment. Whether it is water or solids, if it has essence of poo in it it is equally bad and in fact the water is more mobile. The reason the septic tank captures the solids is that the leach field is intentionally designed with very small gravel to get very high surface area. For purely mechanical reasons the solids, if they get through, will clog up the leach field. That is the reason that septic tanks must be pumped regularly. Cesspools sometimes also need to be pumped but by the time that happens you are screwed because it means that the little cracks are all full of solids and the water can't escape. If you let the septic tank go that far you would be equally screwed, but at least with the septic tank, periodic preventive pumping can maintain the porosity of the leach field indefinitely by preventing solids from ever getting to the leach field.