RESUMO

A hydrogeochemical and isotopic study has been carried out to understand the hydrogeological functioning of a small alluvial aquifer in central Chile in a context of mega-drought and intensive exploitation of its waters. Additionally, two mine tailings dams from porphyry copper mining are situated in the area. The prolonged mega-drought, which has lasted for over thirteen years, has resulted in a significant decrease in rainfall recharge and a drop of up to 50 m in piezometric levels, although no serious groundwater contamination problems have yet been detected, except for a rise in nitrate contents (ranging between 23 and 45 mg/L NO3) attributed to return irrigation. Groundwaters are calcium-bicarbonate and calcium-sodium-bicarbonate in composition. The values of δ18O and δ2H of the alluvial aquifer indicate fractionation by evaporation that would be explained by the recirculation of water that occurs in the agricultural areas of the basin, where the excess irrigation water that go back to the aquifer presents fractionation by evaporation. The δ34S and δ18O of dissolved sulfate point to pyrite oxidation, which could be related to the pyrite present in the copper porphyry and recognized in the Andes Cordillera. The 87Sr/86Sr isotopic values of the alluvial aquifer waters are close to the isotopic fingerprint of the volcanic rocks of the Abanico Formation. However, the water from the wells located further downstream in the basin and close to the tailing dams show δ34S and δ18O of dissolved sulfate and 87Sr/86Sr consistent with Miocene intrusive mineralogies of the copper porphyry type. The groundwater chemistry does not show water seepage from the tailings dam. Therefore, a minor contribution of minerals related to the intrusive rocks is proposed, which would originate from the movement of fine particles by the wind from the dams to the valley floor. The 14C activities indicate that groundwater is recent.

RESUMO

OBJECTIVE: To evaluate the influence of individual spatial units (ie, counties) on the epidemic spread of foot-and-mouth disease (FMD) virus. SAMPLE POPULATION: 163 counties in Uruguay where there was an outbreak of FMD between April 23 and July 11, 2001. PROCEDURE: A geographically referenced database was created, and the distance between counties (13,203 county pairs), road density of counties (163 counties), and time when cases were reported in those counties (11 weeks of the epidemic) were considered to assess global spatial and spatial-temporal autocorrelation, determine the contribution of links connecting pairs of counties with infected animals, and allow us to hypothesize the influence for spread during the epidemic for counties with greater than the mean infective link contributions. RESULTS: Case clusters were indicated by the Moran Iand Mantel tests during the first 6 weeks of the epidemic. Spatial lags between pairs of counties with infected animals revealed case clustering before and after vaccination was implemented. Temporal lags predicted autocorrelation for up to 3 weeks. Link indices identified counties expected to facilitate epidemic spread. If control measures had been implemented in counties with a high index link (identifiable as early as week 1 of the epidemic), they could have prevented (by week 11 of the epidemic) at least 2.5 times as many cases per square kilometer than the same measures implemented in counties with average link indices. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of spatial autocorrelation and infective link indices may identify network conditions that facilitate (or prevent) disease spread.

Assuntos

RESUMO

OBJECTIVE: To explore whether early analysis of spatial data may result in identification of variables associated with epidemic spread of foot and mouth disease. SAMPLE POPULATION: 37 farms with infected cattle (ie, case farms) reported within the first 6 days of the 2001 Uruguayan foot-and-mouth disease epidemic. PROCEDURE: A georeferenced database was created and retrospective analysis was performed on case farm location in relation to farm density, cattle density, farm type (ie, beef vs dairy cattle production), road density, case farm distance to the nearest road, farm size, farm ownership, and day of infection. Mean or median results of 1 to 3 day versus 4 to 6 day spatial data were compared. Spatial-temporal associations were investigated by correlation analysis. RESULTS: Comparison of mean or median values between the first 3 days and days 4 to 6 of the epidemic and results of correlation analysis indicated a significant increase in road density, cattle density, and dairy cattle production and a significant decrease in farm size and case farm distance to the nearest road that developed over time. A route that linked most case farms by the shortest possible distance and also considered significantly associated variables was created. It included 86.1% of all case farms reported by 60 days into the epidemic. CONCLUSIONS AND CLINICAL RELEVANCE: Epidemic direction can be assessed on the basis of road density and other spatial variables as early as 6 days into an epidemic. Epidemic control areas may be more effectively identified if local and regional georeferenced data are considered.

Assuntos