GEOCHEMICAL EVOLUTION OF LACUSTRINE BRINES FROM VARIABLE-SCALE GROUNDWATER CIRCULATION

Evaporative groundwater-fed lakes in the glaciated North American Grea t Plains vary widely in chemistry. A contributing cause is chemical va riability of.source groundwater intercepted by specific lakes, caused in part by differing depths of groundwater circulation. Aqueous chemic al characteristics of 61 lakes and 160 groundwater samples were compar ed for an area where such lakes are common in eastern Montana-western North Dakota. Results indicate that groundwater chemistry varies accor ding to depth in a similar fashion within different aquifers. Lake wat er evaporation from initial groundwater solutions typical of three dep ths was geochemically modeled using PHRQPITZ, based on a Pitzer treatm ent of activities and equilibria. Results show that chemistry of most lake waters in the study area may correspond to that predicted from ev aporation of shallow- and intermediate-depth groundwater, but not of d eep groundwater as postulated in some previous investigations. Lakes i n shallow surface depressions receive water primarily from shallow (lo cal) groundwater flow; lakes located in deep or broad topographic depr essions may additionally receive groundwater from deeper circulation. In the field area studied, relative dominance of anions (sulfate vs. c arbonate) in brines is a signature for inferred depth of source. Also diagnostic is the suite of brine salts formed (Na-SO4-Mg salts for sha llow flow; these plus Na-CO3 salts for intermediate depth flow). Such source signatures will vary from area to area according to depth varia tions in groundwater chemistry and in stratigraphy. Chemical evolution of lake water is a two-stage process, with a groundwater path (influe nced by residence time, depth of circulation, aquifer mineralogy, and related factors) and a surface path (influenced by evaporation rates, lake-aquifer hydraulics, and lake geochemical reactions). Groundwater flow patterns may affect the former set of factors, thereby indirectly controlling lake water chemistry.