drinking water Quality, la manzanilla
Posted by Calzones on October 30, 2010, 3:29 pm
Although I have been a frequent follower of the message board, this is my first posting. My job as an Environmental Health Specialist takes me around the world to assist local governments and private organizations in addressing disease relating to water, wastewater, garbage, mosquitoes, food hygiene, housing, toxics, and rabies control. When I am not working as a water/sanitation/public health advisor, I enjoy my home here in La Manzanilla, and occaisionally get the opportunity to share experience with my own community, local authorities, home-builders and agencies. I have worked closely with the operator of the La Manzanilla water system over the past few years and would like to offer some of my observations, facts and conclusions about our water quality here. In general i can say that we are blessed with good quality water and a responsible, dedicated operator who takes our health very seriously. |
FAQ: Where does La Manzanilla water come from?
A: La Manzanilla depends on groundwater from two drilled wells. One, on the east side of town 150 meters off the entry road is more than 90 feet deep and draws water from an unconfined (not bound betweenhard layers) aquifer. There are no sources of subsurface contamination within 200 feet of the well.
The second well is located adjacent to the soccer field and is more than 100 feet deep in partially confined aquifer. There is one septic system 48 meters feet from the well (the World health standard is 30 meters, the US standard is 60 meters for public wells).
Water quantities in both wells vary with the season and depend on rainy season recharge.
Q: what is the bacteriologic (microbial)quality of the water?
A: Raw water from both wells is tested infrequently (every few months), and results over the past few years have been negative for fecal coliform (of intestinal origin) bacteria. Two samples last year were positive for general coliform(common in soil)bacteria.
Because of those positive results (and because one well is in an unconfined aquifer) chlorine is added to the water in small quantitie to ensure that no disease-causing bacteria are present when the water reaches your tap. Chlorine in small quantities is safe, and used in virtually every large water system in the US and Canada.
Treated water from the distribution network is tested daily for the presence of free chlorine. Except for two tests during the peak of the rainy season this year, the system has maintained a constant residual of free chlorine and therefore has been free of disease causing organisms.
Treated water in the distribution system is also tested for bacteria every month by the health center. Although record keeping has been incomplete, there is no indication of positive coliform bacterial counts in the past few years.
Q. What about back-siphonage during times of low or negative pressure?
A:The system is pressurized by the well pumps(not the storage tank on the side of the hill above the soccer field), and is only pressurized during the day, usually from 7am to 430 pm). When the pumps are shut down, water can siphon back into the main lines from laterals, submerged hoses and cross-connected fixtures. It is possible that a hose could be submerged in highly polluted water which siphons back into the system upon drops in pressure, even through a closed valve.
The presence of free chlorine in the water is a necessary safeguard to protect against bacterial contamination during events of back-siphonage.
Q. What about chemical quality of the water?
A. Because they draw from different aquifers, the two wells differ in chemical quality. The eastside well is moderately high in mineral content and delivers slightly hard water.
Salinity (salt content) varies with the season. Just after the wet season(now), salinity is well within acceptable standards. However as water levels drop in March and april, the salt content rises to as high as 300 parts per million (ppm), well above the US standard of 225 ppm. over the years, there has been an overall trend toward increasing salinity. There is some inconclusive evidence that heavy groundwater withdrawals from an upstream well to supply Tamarindo have increased salt intrusion of the aquifer.
Note that water from the soccer field well normally stays within acceptable standards. Although the water is mixed in the distribution system, generally water from the south side of town is less salty than water from the north side. Salt is of "secondary" concern because it primarily affects esthetic qualities of the water, and is not generally detremental to health (except in people restricted to low salt diets).
The testing for other chemical constituents (metals, phosphates, pesticides) is grossly out of date, and should be repeated soon.
Q. What can we do to help ensure safe water?
A. Lots of things. Avoid submerged hoses and water tank fill lines. Occaisionally add chlorine to our storage tanks (1/2 cup of chlorine bleach per 1000 liters is plenty to disinfect a tank. Pay your water bill. At 50 pesos a month for a basic residece, it is a tremendous bargain. Conserve water, report or fix leaks
Q. What about the sewage system in La Manzanilla?
A. Thats another story completely; as currently operated, the community wastewater system represents a serious health threat to our town, our environment and its guests.
Please know that we (a local committee of concerned residents and health officials)are working at resolving severe operational deficiencies in an otherwise sound well-designed and functional treatment system.
I would be happy to report next week on progress, and discuss the issue either through this forum or at a meeting at the local health center if folks would like more information.
And remember, the first line in preventing foodborne illnes is frequent and thorough handwashing.
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