Mass balance calculations using the verified chemical models predict a spatial distribution of calcite ... All chemical analyses of groundwater used in subsequent calculations have .... All of the minerals in the Aquia react to some degree with groundwater. .... order to quantify these effects, chemical equations that describe the ...
WATER RESOURCES RESEARCH, VOL. 19, NO.2, PAGES 545-558, APRIL 1983
Groundwater Geochemistry and Calcite Cementation of the Aquia Aquifer in Southern Maryland FRANCIS
H.
CHAPELLE
Water Resources Division, U.S. Geological Survey, Towson, Maryland 21204 . The Aquia aquifer in southern Maryland exhibits three regions that are characterized by distinctly different water composition. Region I is about 26 mi (42 km) wide and is parallel to the outcrop area. Water in region I has relatively high calcium (- 30 mg/1) , magnesium (- 10 mg/1) , and bicarbonate (- 150 mg/1) concentrations and relatively low sodium (- 2 mg/1) concentrations . Region II , parallel to and downgradient from region I, is about 24 mi (38 km) wide . Water in region II is characterized by constant bicarbonate, increasing sodium, decreasing calcium , and decreasing magnesium concentrations. Region III is downgradient from region II and is characterized by low calcium (- 2 mg/1) , low magnesium (- I mg/1) , high bicarbonate (- 300 mg/1), and high sodium (- 100 mg/1) concentrations. The major lithologic constituents of the Aquia aquifer are quartz sand (- 55%), glauconite (- 30%), and carbonate shell material (- 8%). Heavy minerals, clay minerals , and finely disseminated lignite make up about 2% of the aquifer material. Calcite cementation is commonly observed in and near the outcrop area of the Aquia . Stoichiometric chemical model s were constructed based on this lithology to describe ~e water chem~s.try of each J:gion. These ~odel s are (region I) (Ca, _X> Mg, )C0 3 ;;=: C3.(aq) 2+ + HC03(aq)-
+ OH(aq) Calcite co,ntaining a significant percentage of magnesium can be treated similarly: (R2)
(Ca,_xMgx)COJ + H20n> ;;=: (l - x)C3.(aq? +
+ xMg(aq)2+ + HC03(aql- + OH(aq) The reaction of C0 2 with water can be written
Development of Working Equations
The lithology of the Aquia aquifer suggests that shell material, carbon dioxide, and glauconite are the aquifer materials that most affect the chemistry of Aquia water. In order to quantify these effects, chemical equations that describe the reaction of each phase with water must be developed. Shell material is commonly composed of at least three phases: calcite, flragonite, and magnesium calcite [Wollast et
(R3)
C02(gl + H20o> ;;=: H(aq)+ + HC03(aq>-
The cation exchange reaction with glauconite acting as the exchange medium can be written X(aq)2+ + Na2 · Glau(ad) ;;=: 2Na(aq/ + X· Glau(ad) where X can be Ca2+ or Mg2+. These equations are based upon idealized mineral compositions and therefore approximate the natural system. (R4)
EX~lANATION
APPROXIMATE LOCATION OF THE AQUIA FORMATION OUTCROP AAEA CM
cto
1
e
10
LOCATION OF WELL. NUMBER IS U.S.G.S . WELL NUMBER .
0
10
20
30
MILES
REGION
I
Well ....- r
...,.......
( ... ./L.)
C•lciuM ........ lum
II
CH Cg 1 Ill Dd 1 130
14
(mg.,;L.)
2t
3.1
( ..../L
13
1.1
Soctiull'l
(mg./L.)
Polaellium
(mg./ L.)
14
1.8
mg.; L.
174
131
Bic•rbon8te Chloride
(mg./L.l
5 .3
1.4
48
2.5
... Ft. Ill
15 3.0 1.8 140 7.8 387 2.4
Sullete
(1119.1 L.)
12
Silica
(mg./ L . )
10
10
11
11.5
17.2
tt.o
7. 7
8.5
1.4
Temperature ( C ") pH
Fig. 4.
8.8
13
Regions I, II, and III in plan view and representative water analyses from each region.
550
CHAPELLE : AQUIA AQUIFER GEOCHEMISTRY
Development of Reaction Models
In the simplified system that was considered in developing the working equations, the source of Ca2+ and Mg2+ ions is dissolution of calcite, aragonite, and magnesium calcite (reactions (R1) and (R2)). The only important source of Na + is the glauconite exchange reaction (reaction (R4)). HC0 3ions are produced by dissolution of calcite, aragonite, and magnesium calcite (reactions (Rl) and (R2)) and by reaction of C0 2 gas with water (reaction (R3)). Various combinations of these working equations can be utilized to account for the water chemistry changes in regions I, II, and III. In region I, sources of Ca2+, Mg2+, and HC0 3- ions are required. Because Na + remains essentially constant in this region, aNa+ source is not required . Therefore the chemistry of region I can be simulated with the working equations: (R1)
CaC03(s) + HzO(l) ~ Ca(aq)2+ + HC03(aq)- + OH(aq)-
(R2)
(Cal-xM&.)COJ = N. Reaction (RIO) was used to represent the dissolution reaction to magnesium calcite in region III . The results of the mass balance calculations are shown in Table 2. These calculations use the water analyses of wells CH-CG-1 , SM-Dd-1 , and SM-Ff-35 (Figure 4) as the water compositions of regions I, II, and III, respectively. The transformed equations of models I, II , and III are shown in Table 2 together with the chemical processes that the equations represent. Table 2 is designed to show the compositional changes of water as it moves along the flow path. The average composition of rainfall in southern Maryland [Junge and Werby, 1958] is acted on by the model I reactions to produce the water composition of region I. Region I water is then acted on by the model II reactions to produce the composition of region II water. Finally, region II water is acted on by the reactions of model III to produce the composition of region III water. The calculations summarized in Table 2 show that dissolution of magnesium calcite and simultaneous precipitation of calcite are required to obtain mass balance in region I. In region II the calculations indicate that cation exchange reactions are the most important chemical processes. Mag-
556
CHAPELLE : AQUIA AQUIFER GEOCHEMISTRY
(")
::r: t'T')
Vl
'--
~ •~o
.., tTl
';};>-
APPROXIMATE OUTCROP · SU8CROP AREA ~ THE AOUIA FORMATION . +10--
_ _.___
0.___
4
LINE ~ EaUAl. THICKNESS ~ CALCITE CEMENTATION CON · TOUR INTERVAL IS 10 FEET
8 12 MILES ____,'-------'
.ASI FlltOM MAIItYLAIC) GEOLOGICAL SUftVEY 1M1 , 1: 250 ,000
77°00'
~·45'
76"1!1'
76"00'
31"oo
31"00'
Fig. 9. Thickness of calcite cementation in the Aquia aquifer.
nesium calcite dissolution and calcite precipitation account for only a smaU portion of the mass balance. This is consistent with the earlier conclusion that region II water chemistry is dominated by cation exchange reactions. In
region III, cation exchange, dissolution of magnesium calcite, and dissolution of calcite are required for mass balance. A comparison of Figures 4 and 9 shows that in region I, where mass balance requires precipitation of calcite, the
TABLE 20 Results of Mass Balance Calculations Water Composition , (moln) X J0- 4 Chemical Processes
Reactions
Water Type meteoric water
dissolve Mg-calcite precipitate calcite dissolve C0 2 gas
(Clloo9M8o.I)COJ + H20;::! Oo9Cll(aq>2+ + OoiM8{aq>2+ + HC01- + OHCaCOJ + H20;::! Cll(aq/ + + HCOJ- + OHC02 + H20 ;::! H+ + HCOJ-
precipitate calcite
dissolve calcite exchange Ca2+ for Na+ exchange Mg2+ for Na+
Mg2+
Na + HC01-
Solid-Phase Products, (moln) x
w-4
0
2030
0
5o20
2030
5200
+46o8
+5020
1202 2800
-3908 0
0 0
2800
000
0
+1008
0
000
0
+5200
0 0
- 3908 +1508
3908 CaC0 3 000
000
39o8 CaC0 3
(j
X
7000 7o00 7000
5o20 5o20 5020
2o30 2o30 2030
000
000
...,>
m
rr-
~
> 0 c:
Modell/
;;
Cll(aq/+ + Na2 Gl,~u(ad) ;::! 2Na+ + Ca Glall(ad)
1.60
5o20
1301
2800
-5 040
M8(aq) + Na2 Glau(ad) ;::! 2Na+ + Mg Glau
1.60
1.00
21.5
2800
0
-4020
+8.4
0
000
c: :;;
(Cao.9M8o.I)COJ + H20(I) + C02;::! Oo9Cll(aq/+ + OoiM8{aq/+ + HCOJCaCOJ + H20 + C02;::! Cll(aq>2+ + 2HC03-
2012
1.06
21.5
29ol6
+Oo52
+0006
0
+ 1.16
000
"'m
1.38
1.06
21.5
27072
-0072
0
0
-1044
0072 CaC0 3
1.38
1.06
21.5
27072
000
000
0072 CaC0 3
>
0
region II water (SM-Dd-1) dissolve Mg-calcite
0
Modell 4608
region I water (CH-Cg-2) exchange Ca 2+ for Na+ exchange Mg2+ for Na+ dissolve Mg-calcite
Ca2+
Change in Water Composition, (moln) X J0- 4 Mg2 + Ca2+ Na+ HC0 3
(Cao.9M8o.I)C03 + H20 + C0 2 ;::! Oo9Cll(aq>2+ + OoiM8{aq> 2+ + 2HC03CaC03 + H20 + C02;::! Cll(aq>2+ + 2HCOJCll(aq)2+ + Na2 Glau(ad);::! 2Nll(aq)+ + Ca Glau
