Page 101 - Hydrogeology Principles and Practice

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84 Chapter Three

Milligan Canyon area, south-west Montana, in the
eastern region of the Northern Rocky Mountain
Province. The area is a broad synclinal basin with
folded carbonate rocks of Palaeozoic and Mesozoic
age on the southern rim, and volcanic breccia and
andesitic lava overlying older, deformed rocks on the
northern rim. The basin is infilled with unconsolid-
ated alluvial and aeolian deposits and Tertiary sedi-
ments of siltstones, limestones and sandstones that
contain deposits of gypsum (CaSO ⋅2H O) and anhy-
4 2
drite (CaSO ). A number of Upper Cretaceous and
4
early Tertiary igneous intrusives are also present.
Groundwater flow is predominantly from west to
east but with a contribution of upward groundwater
flow from the Madison limestone aquifer underlying
the basin. High groundwater yields are obtained
from the Tertiary basin.
The same major ion analyses for the Milligan
Canyon area are also presented as trilinear and Durov
diagrams in Figs 3.8 and 3.9. With these methods of
graphical presentation, the concentrations of indivi-
dual samples are plotted as percentages of the total
cation and/or anion concentrations, such that sam-
ples with very different total ionic concentrations can
occupy the same position in the diagrams. Also, with
the trilinear diagram (Fig. 3.8), samples that plot on
a straight line within the central diamond field
represent mixing of groundwaters between two end-
member solutions, for example freshwater and saline
water. Further hydrochemical interpretations can be
obtained from the Durov diagram (Fig. 3.9). Lines
from the central square field can be extended to the
adjacent scaled rectangles to allow for representation
in terms of two further parameters.
The next step in the hydrochemical interpretation
after plotting the chemical data in a variety of ways is
to identify the hydrochemical facies present and to
prepare maps and cross-sections to show the regional
distribution of water types. In the example of ground-
waters in the Milligan Canyon area, Fig. 3.10 shows

Fig. 3.6 (left) Schoeller (semi-logarithmic) diagram illustrating
near-surface groundwater chemistry in crystalline rocks. Symbols
on the plot indicate rock type: QZ, quartzite; GA, gabbro; GR,
granite; M, marble. Analyses shown are for Houghton County,
Michigan; Thompson, Manitoba; New Mexico (mixture of native
and injected surface waters) and California. After compilation by
Trainer (1988).