In the Mezquital valley, untreated wastewater
(45 m3 s
-1
) from Mexico City is used for the irrigation of
around 900 km2 of agricultural soil. High concentrations
of metals including methylmercury (3.8±2.5 ng l-1
) and
lead (0.16±0.05 mg l-1
) were measured in anoxic
wastewater canals. Downstream, dissolved, and particulate
polymetallic (Hg, Pb, Cr…) concentrations decreased
by factors 10 to 1,000 in the Tula River (which
received a mix of fresh and wastewater) due to the
dilution and oxidation of surface water, and to the
decrease of contaminants concentration in wastewater
downstream irrigated soils. However, dissolved and
particulate methylmercury concentrations (0.06 to
0.33 ng l-1 and 1.6 to 4.5 µg kg-1
, respectively)
remained elevated in comparison to other natural
hydrosystems. The monitoring of an irrigation event
and the distribution of metals in a soil profile irrigated
for more than 80 years showed that metals were retained
in the draining tilled layer. The oxic conditions and
slightly acidic pH (~6.5) in this layer were found favorable
for metal adsorption and co-precipitation with
redox-sensitive elements (Fe, Mn) and suggestively for
mercury demethylation. In the downstream Tula River
and groundwater, almost all metallic concentrations
remained below guideline thresholds. Only, dissolved
As and Pb concentrations remained two to five times
above thresholds for drinking water, highlighting a potential
health risk for approximately 500,000 people
who use groundwater as water supply.