Attenuation of arsenic in karst subterranean stream and analysis of its influence factors: A case study at Lihu subterranean stream,

Abstract

Arsenic (As) pollutants are serious threat to water ecological security and human health, especially in karst areas because of their unique hydrogeological characteristics. Physical-chemical analyses of karst water and its sediments at the Lihu subterranean stream, southwest China, were conducted by ICP-MS and XRF to elucidate the reaction mechanisms of arsenic in karst subterranean streams. The results show that inorganic arsenic comprise most of the total arsenic, while organic arsenic including monomethylated arsenic (MMA) and dimethyl arsenic (DMA) are not detected or infinitesimal. The reducing environment in the subterranean stream makes As(III) dominant and accounts for 53% of the inorganic species. Adsorptive behaviour of arsenic occurred and the removal rates of As, As(III) and As(V) in the Lihu subterranean stream are 51%, 36% and 59% respectively after a 25.6 km underground distance. To find out the main influencing factors on arsenic adsorptive process in this underground river, principal component analysis in SPSS and Minitab were applied. Seven main factors, i.e. sediment Fe (Fesed), sediment Al (Alsed), sediment Ca (Cased), particulate organic matter (POM), sediment Mn (Mnsed), water Ca2+ (Ca2+) and water HCO3– (HCO3–) are extracted from thirteen indicators. The rank of those factors for total arsenic and As(III) is Ca_sed > Fe_sed > Ca2+ > POM > Mn_sed > Alsed > HCO3 –, while it is Fesed > Cased > Ca2+ > Mnsed > POM > Alsed > HCO3 – for As(V). Of these seven factors, Fesed, Alsed, Cased, POM, Mnsed and Ca2+ are promoting factors for arsenic adsorption while HCO3– is an inhibiting factor. Calcium and bicarbonate turn out to be the main influencing factors for water arsenic adsorption in the study area, largely because the high calcium and alkaline values in karst water. This finding is an obvious distinction compared with the research findings at a non-karst area.