Abstract :
Many owners of domestic wells shock chlorinate their wells to treat for bacterial contamination or control bad odors from
sulfides. Analysis of well water with four wells from Fallon, Nevada, showed that following recommended procedures for shock
chlorinating wells can cause large, short-lasting increases in trace-element concentrations in ground water, particularly for Cu, Fe,
Pb, and Zn. Lead concentrations increased up to 745 fold between samples collected just before the well was shock chlorinated and
the first sample collected 22–24 h later; Zn concentrations increased up to 252 fold, Fe concentrations increased up to 114 fold, and
Cu concentrations increased up to 29 fold. Lead concentrations returned to near background levels following pumping of about one
casing volume, however, in one well an estimated 120 mg of excess Pb were pumped before concentrations returned to
prechlorination levels. Total Pb concentrations were much greater than filtered (0.45 μm) concentrations, indicating the excess Pb
is principally particulate. Recommended procedures for purging treated wells following shock chlorination may be ineffective
because a strong NaOCl solution can remain in the casing above the pump even following extended pumping. Only small changes
in gross alpha and beta radioactivity occurred following shock chlorination. USEPA has not promulgated drinking-water standards
for 210Pb, however, measured 210Pb activities in the study area typically were less than the Canadian Maximum Acceptable
Concentration of 100 mBq/L. By consuming well water shortly after shock chlorination the public may inadvertently be exposed to
levels of Pb, and possibly 210Pb, that exceed drinking-water standards.
Keywords :
Shock chlorination , trace elements , Lead-210 , lead