Major and Trace Ions in Ganges Brahmaputra Water
Introduction & Motivation
I propose the analysis of dissolved major cation and trace metal concentrations by inductively coupled plasma mass spectrometry (ICP-MS) on Ganges-Brahmaputra time series samples. Dissolved major cation concentrations (Na+, K+, Mg2+, and Ca2+), combined with dissolved major anions (F-, Cl-, SO42-; currently measured using ion chromatography) and dissolved inorganic carbon (DIC), fully constrain the source (i.e. carbonate or silicate rock) and quantity of chemical erosion in the Himalayas and in the floodplain. Quantifying this erosion is important both globally, as a sink of atmospheric CO2, and locally, to determine the state of agriculturally important floodplain soils. Additionally, quantifying trace metal flux is crucial both as a source to the ocean and for determining water quality for floodplain inhabitants, as contamination (i.e. of arsenic) is a known issue in this region.
While previous chemical erosion budgets exist1-3, high-resolution time series measurements are necessary to correlate lithologic and climatic control and to determine inter-seasonal and interannual variability. For example, previous research shows that total dissolved solid (TDS) discharge is highly correlated with water discharge4 and that chemical weathering is potentially controlled by climatic factors such as precipitation and temperature5,6. Here, my samples span two monsoons at weekly resolution and one dry season at monthly resolution on both the Ganges and Brahmaputra rivers, just upstream of the confluence (Figure 1). In addition to dissolved analysis proposed, my samples include both the suspended and bedload sediments necessary to quantify physical erosion, and these are currently being analyzed for major and trace element concentrations. This combination of chemical and physical erosion rates will allow me to
Sediments are exposed to chemical conditions during riverine transport that favor dissolution of carbonate and silicate rock by reaction with atmosphere-derived CO2. Over geologic timescales, this drawdown and subsequent burial (as biogenic CaCO3) in marine sediments is a major control on atmospheric CO2 levels. To make quantitative estimates of this process as an atmospheric carbon dioxide sink, one must also know the relative proportion of carbonate and silicate rock weathered: all DIC from silicate rock weathering is derived from the atmosphere, while half of the DIC from carbonate weathering comes from the rock itself. Fortunately, weathering of these two rock types releases a unique signature of corresponding cations, and analysis of these cations can be used to both determine the source and quantity of rock weathering.In addition to this lithologic control, the Himalaya range is subject to strong climatic forcing from the Indian Summer Monsoon, causing a roughly ten-fold increase in discharge between wet and dry seasons. Previous results show that discharge is the primary control on chemical erosion rates by physically removing top bedrock layers and exposing fresh, highly erodible bedrock below4. Thus, future changes in monsoon strength due to increased atmospheric CO2 could induce a feedback loop by altering the amount of CO2 drawdown by chemical erosion. Current annual estimates place Ganges-Brahmaputra TDS export at roughly 130 million tons, but are based on either campaign style sampling or coarse time series lasting less than one year1,7, and my proposed measurements will determine this annual flux more accurately.
ICP-MS analysis of trace metal concentrations and their temporal variability will offer great insight into the sustainability of Bangladeshi agriculture and water quality, as the Ganges-Brahmaputra delta region contains millions of rice farms and provides food for nearly 200 million Bangladeshi citizens. Previous campaign-style samples indicate that soil stocks in this region are not in steady state – i.e. more soil is being eroded than can be resupplied. By comparing the dissolved concentrations of soluble elements proposed here with corresponding particulate concentrations, I will assess the validity of this preliminary result over annual timescales.
Additionally, it is well known that groundwater supply in this region exposes millions of people to toxic levels of Arsenic8. If there is significant exchange between groundwater and Ganges-Brahmaputra river water, either naturally or anthropogenically due to irrigated rice patties, I expect elevated arsenic levels in river water9. This effect should be more pronounced during the dry season, as overland monsoon rainwater will dilute any arsenic signal. Thus, time series analysis of arsenic and other trace metals will determine floodplain water quality dynamics as well as trace metal fluxes to the ocean.
Measurements made using the COI grant will allow me to constrain many processes related to the Ganges-Brahmaputra river system, including: physical and chemical erosion of Himalayan bedrock (and corresponding CO2 drawdown), assessing future delta soil stocks, and trace metal flux and corresponding delta water quality. Results from these proposed measurements are directly related to my thesis, as my central goal is to understand the climatic and geologic controls on seasonal variability of riverine carbon export.
These measurements constitute one aspect of a larger Ganges-Brahmaputra time series project and results will be included in a future NSF proposal. Budget WHOI ICP-MS facility (Element2) for 1 day @ $1,240 per day
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