Assessment of heavy metals in sediment of Aguamilpa Dam, Mexico.

The Aguamilpa Dam is part of the reservoir cascade system formed by four reservoirs in the middle and lower part of the Santiago River. For decades, this system has received urban and industrial wastewater from the metropolitan area of Guadalajara and the runoff of agricultural fields located in the river basin. The present study was carried out to obtain a preliminary assessment on the concentration distribution of heavy metals (Al, Ba, Cd, Cr, Cu, Fe, Hg, Mg, Ni, Pb, and Zn) in surface sediments of the Aguamilpa reservoir collected from 10 sampling stations. The metal concentrations (mg kg(-1)) in the sampling stations ranged as follows: Al, 27,600-7760; Ba, 190.0-15.9; Cd, 0.27-0.02; Cr, 18.30-0.22; Cu, 60.80-0.79; Fe, 15,900-4740; Hg, 0.04-0.01; Mg, 7590-8.05; Ni, 189.00-0.24; Pb, 13.6-1.64; and Zn, 51.8-14.8. Significant spatial variation in concentrations was observed for Al, Fe, and Pb. Sediment pollution was evaluated using the enrichment factor, the geo-accumulation index, the pollution load index, and sediment quality guidelines. Based on geo-accumulation and pollution load indexes, Aguamilpa sediments were found, in some sampling stations, as unpolluted to moderately polluted with Ni, Cd, Cu, and Mg. Enrichment factors showed that Cd is highly related to agricultural activities that take place in the surrounding areas of the Aguamilpa reservoir. Despite these results, none of the heavy metals evaluated exceeded international concentrations limits, indicating that the Aguamilpa reservoir surface sediments are not contaminated.

Pollution by organochlorine pesticides in Navachiste-Macapule, Sinaloa, Mexico.

The lagoon system of Navachiste-Macapule is located in northern Mexico, in the state of Sinaloa, with an area of 24,000 ha. The main economic activity in the area is agriculture, and the lagoon lies next to the irrigation district ID-063 which covers 116,615 ha. The purpose of this study is to evaluate the levels of pollution generated by organochlorine pesticides (OC) in the surface sediments of the lagoon and in the agricultural drains of the ID-063 that are supposedly transported into the system as a result of agricultural activities and runoff from adjacent land. For this, between 2006 and 2007, 45 surface sediment samples were collected (warm dry, rainy and cold dry) from 15 sampling sites, during the three climatic seasons. Of these, eight were located inside the lagoon in marine conditions (salinity >31 PSU) and seven in the agricultural drains of the ID-063 in freshwater conditions (salinity <5 PSU). The average concentration of the OC in the sediments was 44.75 ng g(-1), among which the group of the alicyclic compounds presented the greatest concentrations. The average value of the total organic carbon (TOC) in the sediments of the system was 0.90%. The sediments collected inside the lagoon had an average OC concentration of 18.97 ng g(-1), and the predominant type of sediment was fine to very fine sand. The average OC concentration in the sediments collected in the agricultural drains was 75.69 ng g(-1), where fine sediments (silt) were predominant. The presence of methoxychlor, endrin and heptachlor suggested that these compounds were continuously used in the system, even though their use is forbidden in Mexico.

Effect of mono and divalent salts on the conformation and composition of a humic acid and on atrazine adsorption.

We investigated the effects of sodium and calcium chlorides on the conformation and composition of a purified Aldrich humic acid (PAHA), as well as on the adsorption of atrazine. The PAHA was treated with 1, 10, and 100 mM NaCl, CaCl2, or a mixture of NaCl and CaCl2 (molar ratio 5:1) at pH 7.5 and 8.5. The conformation of treated PAHA was characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM) and spectral changes of functional groups of PAHA by Fourier transform infrared spectroscopy (FTIR). AFM and TEM images showed an increase in the aggregation of the PAHA as salinity increased. FTIR spectra revealed that changes in the aggregation of the PAHA were principally due to the formation of bridged interactions between calcium and carboxylate groups in the PAHA. The adsorption of atrazine on > 0.45 µm PAHA decreased as salt concentrations and pH increased. This reduction of atrazine adsorption was explained by the decrease in available adsorption sites due to agglomeration of PAHA.