Mallard Pond 



Climatic setting. The net annual precipitation balance for Mallard 

 Pond derived from the MAPS database is -23 inches, toward the less 

 evaporative end for sites in this class. Based on analyses of 

 evaporation pan data, this is a considerable underestimate Mallard 

 Pond's water deficit, and points out the need to view the MAPS 

 estimates only as relative indicators of net site precipitation. 

 Mean monthly precipitation for the Medicine Lake climatic station 

 shows a pronounced June maximum. Annual totals for this station 

 show apparent periodicity with a 5 to 7 year cycle. Throughout the 

 1980s, the 3-year running average remained below (often well below) 

 the long-term average, recovering in the early 1990s. In 1993 (the 

 year before sampling) precipitation exceeded the long-term average 

 by more than 1 inch. 



Geologic setting. Mallard Pond occupies an ice-block depression 

 (kettle) in the sequence of pitted late Pleistocene outwash sand 

 and gravel forming the Clear Lake aquifer. Glacial sediments are 

 underlain at depth by the early Tertiary Fort Union Formation. 



Hydrologic type. Mallard Pond is a glacial depression lacking 

 significant surface water inflow but possessing surface water (and 

 probably ground-water) outflow. Inflow to the pond is primarily 

 discharging ground water from the Clear Lake aquifer. 



Basin characteristics. Mallard Pond has only a minimal surface 

 catchment; the catchment to wetland area ratio ranks toward the 

 lower end of the range shown by sample sites of the Great Plains. 

 The pond is surrounded on three sides by steep kettle walls; on the 

 low-gradient west side, the pond drains by discontinuous(?) 

 channels and ground-water flow toward the Lake Creek channel. The 

 depth of the pond is unknown but believed to be shallow. The WET47 

 site is on the upgradient side of Mallard Pond with respect to the 

 regional Clear Lake aquifer flow system; important spatial water 

 quality gradients can exist within this small pond, as shown by 

 previous sampling (see below). 



Water chemistry. At the time of sampling. Mallard Pond contained 

 "oligosaline" sodium-sulfate water of moderately alkaline pH and 

 depressed dissolved oxygen content. Substantial calcium remained 

 in solution and mineral equilibria indicated slight oversaturation 

 with calcite and aragonite. The sample was collected in an area 

 of probable ground-water discharge and is interpreted to have been 

 out of equilibrium with surface conditions (see below). The 

 ammonium concentration was the highest of sites in this 

 classification, and among the highest in the sample set; 

 significant nitrate (among the four highest sample sites) was also 

 present. Phosphorous (total and orthophosphorous) and total 

 organic carbon concentrations ranked mid-range among sites of the 

 glaciated Great Plains, but toward the upper end of the ranges 

 shown by sites possessing surface water outlets. 



