Seasonal Variations of Chemical Compositions of Lake and Inflowing River Water from Nam Co Catchment — a Three-Year Monitoring Study

Abstract:

Major ionic compositions of water are of importance in a catchment, because they reflect sources of materials deposited in a lake and the controlling mechanisms. Especially Ca²⁺, Mg²⁺ and HCO₃^- are of interest, because they influence the process of carbonate precipitation in a lake. In this study, we present the seasonal variations of chemical compositions of lake and inflowing river water from Nam Co area.

Two sites (T1 and T2) were selected for regular water sampling in Nam Co, they were located in eastern small basin (T1, water depth ~57m) and center area of the main basin (T2, water depth ~93m), respectively. Totally twenty one rivers were selected for water sampling around the lake. Water samplings were conducted in T1 and T2 stations with 10m depth interval from the top to the bottom of water column. The samples were taken almost in every month from May to December of 2011-2013.

The results showed significant increase of major ions during post-mosoon period (late October) than pre-monsoon period (mid-May). In lake water, Mg²⁺ increased by 46.84% and 46.95% on average in two sites, while Ca²⁺ increased by 67.02% and 75.11% on average, respectively. HCO₃^- increased 27.61% and 25.02% in two sites, respectively. In river water, most ions also showed the similar trends, Mg²⁺ and Ca²⁺ increased by 40.55% and 33.20% averagely. HCO₃^- and SO₄^2- increased 36.81% and 28.48%.

Total dissolved solid (TDS) was used to evaluate seasonal variations of water chemistry. It is indicated both lake and river water show similar patterns. The lowest concentration of lake and river water (T2, ~1220mg/l; river average, ~95mg/l) was appeared in spring (mid-May) after the lake ice was melted. Then the TDS increased very quickly by ~10.7% and remained at a relatively stable level until mid-September, another increase of TDS occurred afterwards with the highest concentration (T2, ~1460mg/l; river average, ~145mg/l) in mid-November. During the stagnation period, the TDS of lake water decreased by 11.0%, This decrease was mainly driven by the decrease of Mg²⁺, Ca²⁺ and HCO₃^-, suggesting precipitation of carbonates within the lake.

Temperature, monsoonal precipitation, chemical weathering, evaporation, carbonate precipitation within the lake might be the main factors controlling the seasonal variations of water chemistry in Nam Co catchment.