Environmental Engineering Reference
In-Depth Information
Snow's investigation included a follow-up on each death. He spotted the loca-
tion of each on a map with relation to the Broad Street well and inquired of the
work and activities of each person, their habits and customs, and source of drink-
ing water. The one common factor was consumption of water from the Broad
Street well. With this information in hand, he convinced the Board of Guardians
of St. James Parish to have the handle of the pump removed, and the epidemic
was brought under control.
A survey was made to determine the cause and source of the epidemic. The
house at 40 Broad Street nearest the well was suspected as the source; there had
been four fatal cases of cholera at the house. A privy emptying into a cesspool,
which served more like a tank, overflowed to a drain passing close to the well.
On further investigation, including excavations, it was found that the Broad
Street well was a brick-lined dug well with a domed brick top 3 feet, 6 inches
below the street. The well was 28 feet, 10 inches deep and 6 feet in diameter,
and contained 7 feet, 6 inches of water. The house drain, 12 inches wide with
brick sides 12 inches high and stone slab top and bottom, passed within 2 feet, 8
inches of the brick lining of the well. The drain, on a very flat grade, was 9 feet,
2 inches above the water level in the well and led to a sewer. The mortar joints
of the well lining and the drain were completely disintegrated. It was found on
inspection after excavation that the drain was like a “sieve and through which
house drainage water must have percolated for a considerable period” into the
well, as indicated by black deposits and washout of fine sand. The drain received
wastewater from 40 Broad Street in addition to the overflow from a cesspool in
the basement, over which there was a privy.
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In another study in 1854, Snow found that a low incidence (37 per 10,000
residences) of cholera fatalities occurred in one part of London supplied by the
Lamberth Company with water from the River Lea, a tributary of the River
Thames, with an intake more than 38 miles upstream from London. People
supplied by the Southwark & Vauxhall Company received water taken from
the heavily wastewater-polluted Thames River, opposite the location of Parlia-
ment, with a very high incidence of cholera and a death rate of 315 per 10,000
residences. Snow compared the income, living conditions, work, and other char-
acteristics of the people in the two areas and found that source of water was the
main variable and, hence, the cause of the illness. The study involved approxi-
mately 300,000 people and laid the basis for future epidemiologic studies.
Today, John Snow is considered the epidemiological giant of his time. How-
ever, his views on the transmission of cholera did not go unchallenged during his
active investigations. William Farr, a professional epidemiologist, was lukewarm
to Snow's findings of 1849 and, although he accepted that an association existed
between cholera illness and the south district water supply of London, clung to
the view that the cholera epidemic of 1849 was responsible to “spread by atmo-
spheric vapours” and the consequences of the lower elevation of water pipes in
the soil carrying water from the lower Thames as opposed to that of the upstream
region.
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Farr also contended that the cholera agent was heavier than water and,
