Hydrology and chemistry of groundwater and seasonal ponds in the Atlantic Coastal Plain in Delaware, USA

Journa! ¢~l"Hydrology, t 41 (1993) 157-178 Elsevier Science Publishers B,V., A m s t e r d a m

157

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Hydrology and chemistry of groundwater and seasonal ponds in the Atlantic Coastal Plain in Delaware, USA Patrick J. Phi!lips" and Robert J. Shedlock b ~'Water Resources Division, U.S. Geologic~¢l Survey, James T. FokT Courthouse 445 Broadu'ay. Box 1669, Aii.'aur, N Y 12201. USA Water Resources Dil,ision, U.S. Geological Survey. 208 Carroll Building, 8600 LaSalle Road, Towson. MD 21286. USA

ABSTRACT Phillips, P,J. and Shedlock. R,J,, 1993. Hydrology and ct~t:mistry of groundwater and seasonal ponds in the Atlantic Coastal Plain in Delaware, USA. J. HydrO., 141:157-178. The hydrochemistry of small seasonal ponds was investig~,~ed by studying relations between groundwater and surface water in a forested Coastal Plain drainage b~sin. Observation of changes in the water table in a series of wells equipped with automatic water-level rec~rders showed that the relation between water-table configuration and basin topography changes seasonally, and particularly in response to spring recharge. Furthermore, in this study area the water table is not a su~bdued expression of the land surface topography, as is commonly assumed. During the summer and tan months, a water-table trough underlies sandy ridges separating the seasonal ponds, and maximum water-table altitudes prevail in the sediments beneath the dry pond bottoms. As the ponds fill with water during tj~e winter, maximum water-table altitudes shift to the upland-margin zone adjacent to the seasonal ponds. Increases in pond stage are associated with the development of transient water-table mounds at the upland-margin wells during the spring. The importance of small local-flow ,sy~iems adjacent to the seasonal ponds aiso is shown by the similarities in the chemistry of the shallow groundwater in the upland margin and water in the seasonal ponds, The upland-margin and snrface-water samples kave low pH (generally less than 5,0), and contain large concentrations of dissolved aluminum (generally more than 100 fig 1- ~!, and low bicarbonate concentrations (2 mg I i or less). In contrast, the paris of the surficial aquifer ~hat do not experience transient mounding have higher pH and larger concentrations of bicarbonate, The~e results suggest that an understanding of the hydrochemistry of seasonally ponded wetlands requires in~'nsive study of the adjacent shallow groundwater-flow syslero.

INTRODUCTION The hydrology and hydrochemistry of lakes and wetlands have bee~l shown to be strongly influenced by the adjacent groundwater system (Winter 1983, Correspondence to: P.J. Phillips, Water Resources Division, U.S, Geological Survey, James T. Folcy Courlhouse~ 445 Broadway, Box 1669, Alblmy, NY 12201, USA.

0022-1694193/$06.00

~! i993 ...... Elsevier Science Publishers B.V. All rights reserved

t58

P.,I PtlII,I,IPSAND R J StlFI)IOUK

1986; Cherkauer and Zager, 1989; and Keuoyer and Anderson, 1989). These studies have shown that the grotmdwater-flow patterns adjacent to surfacewater bodies are transient and can strongly influence groundwaler discharge to, and water chemistry ol; lakes and wetlands. Although documentation of transient water-table phenomena has been made for areas in the midwestern and northeastern United States, (see Anderson and Munter, 1981; Shedlock et al., 1989), few similar studies of the small seasonal ponds in the Coastal Plain environment have been done. The central Delmarva Peninsula in eastern Maryhmd and Delaware contains hundreds of small seasonal ponds. These shallow ponds are generally less than I ha in size and, in most years, have no standing water from midsummer to early winter. Bachman and Katz (1986) observed that in small eastern Maryland streams, short groundwater-flow paths adjacent to streams could be the cause of elevated aluminum concentrations and low pH levels in the streams. If transient mounds form adjacent to the ponds, the pond chemistry could be influenced by short groundwater-flow paths. Hence, these seasonal ponds could be susceptible to the effects of low-pH precipitation. This paper documents the results of an investigation of the hydrologic relations between seasonal ponds and the surrounding shallow groundwaterflow system in a Coastal Plain setting in northern Delaware, and shows the importance of these relations for pond and groundwater chemi,.-t.ry. Because these seasonal ponds occur along a drainage divide, these results may also be useful for understanding groundwater and surface-water interactions in headwater streams. LOCATION AND GEOLOGIC SETTING

The study site is located in Blackbird State Forest, near Vandyke, Dehware (Fig. I). The study sile is a relatively undisturbcd, fores~.cd, headwaler area with ttUllleroLiss(,~aSOl~lal ponds~ Many of tile scaso,aal ponds in the study area col]l~till |'are plant and animal species, and the biota of these ponds is monitored by State conservation officials. The topography at Blackbird State Forest is similar to other hummocky, pc,orly drained areas in the central uplands of the Delmarva Peninsula (U.S. DeI~artment of Agriculture, 1970), The study area lies along a broad, poorly dissected drainage divide Ihat separates the Sassafras and Clmster Rivers ...... two maj,~r Iributaries that flow westw!trd to the Chesapeake Bay (Fig. i). Elevations at Blackbird State Forest range from less than 22,5 m above sea-level (m.a.s.l.) at the southern end of the study area to just above 27 m along the ridge between Pond l and Pond 2 (Fig. 2). "rite tow, wet area southeast of Pond 3 forms the headwaters of a first-order stream and is tile nearest stream channel to the study area, The

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F'ig. t. Maps of (A) general location of study area at Blackbird State Forest, Delaware, and (B) location of study ~:rea relative to !he Chester River, the Sassafras River and the drainage divide between the Chesapeake and Delaware bays.

bottoi~as of the ponds range in elevation from about 23 to 24 m.a.s.l. Buttonbush and grasses are the dominant vegetation in the ponds. A few scattered sweet gum trees and maple saplings grow along the upper edges of the ponds. The uplands separating the seasonal ponds differ in width and in elevation above the ponds. Much of the study area is less thai,, 1.5 m above the m~:ximum stage observed in the ponds during the study. The most prominent upland lies between Pond 2 and Pond 1, and consists of a r.orthwest-southeast-trending ridge up to 100 in wide and 3-4 m higher thar~ the maximum stages measuled in the ponds (Fig. 2). This upland narrows to ~pproximately 60 m wide to the southeast, where it separates Pond 1 and Pond 8. A narrower upland ridge less than 30 m wide and 2 m higher than Pond 1 separates Pond 1 from Pond 3 and continues northwest of well D-06 for approximately 90 m. Pond 1 ties in a large closed depression that extends to the northwest. The elevation of much of the area between Pond 1 and Pond 6 is less tha:'l 1.5 m above the maximum stage of Pond I. With the exception of Pond 3, all the ponds lie in closed depressions, with no surface~water inlets or drainage'- At

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P.J. PHIL[.IPH AND R.J .";ItE|)I.OUK

BLACKBIRD STATE FOREST WELL NETWORK

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Upl.~nde abo~e 245 me~ers in elevation Welt8 equipped with f¢ouriy water -level recordecs

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[:ig, 2. Mttps ofl~*~Hi~w~~l~tl well tleiw~'k I't~r stully llretl ~1 Blackbird Slale Ft~rest, Delaware showing (A) section A .A' ~i~'~fi~n trdce ~md (P,) :~e¢1i¢~1~B | ) ' ttlld seclion C - C ' traces.

its maximum stage, Pond 3 drains to the south through a small surface-water ~,,~ttlet adjacent to well E-03. The shallow aquifer in tile study area consists of the Pensauken Formation, a predominantly sandy deposit, which is 7,5-9.0 m thick in this tu'ea (Owens and Minttrd, 1979)~The Pen:~akcn l:orm~tion overlies Ihe silt, sand and clay of the Calvert Form~ttion (Pickett and Benson, 1977). The stratigraphy of lhe near-surface sediments from Pond 2 south through Pond 3, shown in Fig. 3, is based on test-hole data and cores collected in the uplands ami in the bottoms of the ponds. The uplands around the ponds are

IIYDROLOGYANDCHEI~tlSTRYOFGROUNDWATERANDSEASONALPONDS North

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~!i,t

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--~=-- Oct, 26. 1987

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Fig. 3, Hydrogeologic

section through Blackbird State Forest si;owing sediments, screened intervals of water-table wells, and water-table configuration for 26 October 1977,(SeeFig. 2 for location of section

trace.) underlain by sand and thin beds of clay and silt. ]'fie deposits beneath the ponds consist of clay, silt, and sand overlain by a thin peat layer, which thickens toward the center of the ponds. The peat layers range in thickness from approximately l cm to mole than 60cm. The clay layers beneath the ponds are generally 3 m or greater in thickness. The clay l~yer beneath Pond 6 is about 2 m thick and is underlain by sand. The upper edges of some of the ponds are underlain by sandy sediments. For example, the s'~uthern edge of Pond 1 and the northeastern edge of Pond 8 are underlain by ~and, and wells E-10 and E-II were screened in these sediments. METHODS Shallow water-table wells were installed to observe water-table changes in the ponds, in the uplands adjacent to the ponds, and in the broad :lpland ridges between the ponds (Figs. 2 and 3). Most of the shallow wells were located along a line between Ponds 2 and 3, because this line parallels the

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region~fl topographic slope of the area. The transect from Pond 2 to Pond 3 is representative of the topographic, geomorphic, and vegetation settings in lhe study area. Most of the [brest is inaccessible to drill rigs; therefore, these wells were either hand driven or installed in holes created by vibracoring aluminum irrigation pipe into saturated sediments (Thompson et al., 1991). To de~ermine the hydrologic relation between the seasonal ponds and the surrounding groundwater-flow system, the wells were completed just below the expected annaal minimum water-table altitude with screens less than ! m long. Hand-driven wells are made of stainless steel casing with wire wound screens. Casing for all other wells is polyvinylchloride plastic pipe with perforated screens. Two shallow wells were installed off the main transect in sandy sediments beneath Ponds 6 and 8. Three supplemental deep wells screened near the base of the surficial aquifer were located at sites accessible to a drill rig. Water-level gages were established in Pol~.~s 1, 2, and 3 by driving perforated metal pipes into the sediments near the centers of the ponds. Because the perlbrations extended fi'om 0.6 m below land surface to 0.3 m above land surface, the stage measurements reflect surface-water stage during the winter and spring, and the groundwater altitude of the sediments during the summer and fall. The transient nature of the groundwater and surface-water systems was studied by measuring hourly water-level changes in wells M-08, R-06, M-05, M-04, and Ponds 2 and 1 with automatic water-level recorders from April 1986 through July 1991. Wells R-07, E-03, and Pond 3 were equipped with hourly water-level recorders from April 1986 through November 1988. Water levels at the remaining wells were measured approximately once a month, and some daily water levels were measured at well E-10 during individual storms. Although water-table altitudes and surface-w~ter slages for the period from I October 1987 through 30 September 1988 (the 1988 wat,~r ye~lr) were used tbr the I~llowing analysis~ simil~u~phenon~ena were observed during most of the 5 years ~f w~lleroleveldau~ coileclion. To relate w,~fler-level and water-chemistry data to local topography, distance from a seasonal pond, and hydrogeology, the wells were divided into ik~urcategories (Fig. 4). Three of the categories include wells screened near the water table, and the fourth category includes the wells screened close to the base of the surficial aquifer. These well categories were incorporated into the well name by using ~1le|ller pretix~ Hence, the 'edge' wells arc designaled by lhe prefix E, "matgi~f wells by the prefix M, "ridge' wells by the prefix R, and ~deep' wells by the pretix D. The first category includes edge wells, which are completed in sanely sediments underlying the bottom of the .~asonal ponds. Although welt E-03

IIYI)ROI OGY AND Ct|EMISTRY OF ~3ROUNDWATER AND SEASONAL PONDS

163

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Fig, 4. Schemalic cross-section s h o w i n g relt*~Jons of edge, m a r g i n , and ridge zones to t o p o g r a p h y a n d p r n x i m i l y to seasonal pond.

lies to the south of the margin of Pored 3, it was included with the edge wells because well E-03 is located on a nar,row, marshy neck of land north of the headwaters of the stream draining the l'~x-est.The second category includes the margin wells, located in the upland tbrest at elevations just above the margin of the seasonal pond and less than 45 m f~,'omthe edge of the ponds. These wells lie inside the large closed depressions partly occupied by the seasonal ponds. The third category, ridge wells, includes two wells that are more than 60 m frorn seasonal ponds, and are located o n or adjacent to, the ridge that lies between Ponds 2 and 1. The fourth category is the deep wells, which includes the four wells screened below the vicinity of the water table. Three of the four deep wells are screened 4.5-6.0 m below the water table near the base of the Pensauken Formation. Well D-09 is compi~'ted in sand" sediments beneath the 3 m thick layer of clay and silt that underlies seasonal Pond 6. Water samples were collected at all of the groundwater wells, in six seasonal ponds, and in the stream at the southern end of the forest in March 1988. Water samples were collected at selected wells and seasonal ponds during the summer and fall of 1987 and fall of 1988. Wells were purg:2d of three casing volumes, and pH, specific conductance, dissolved oxygen, c~nd temperature were monitored to assess stability before sampling. Alkalinity concentrations, pH, and water temperatt, re were measured in the field and laboratory. Alkalinity concenlrations were measured using a modified Gr,~'n titration. Water samples were analyzed at the U.S. Geological Survey National Water Quality Laboratory in Arvada, CO, using analytical techniques o~.,.tlinedby Skougstand et al. (1979). All samples were analyzed for concentrations of major cations, anions, dissolved organic carbon, dissolved aluminum, and dissolved iron, Water s~mp!es collected for enncentration~ of iron, alum.inure, major cations, and m~0or anions were passed through a 0.1 itm tilter pri~r to

PJ, PHil liPS A~I) R I glll~t)l O(K

164

analysis. Use of a 0.1 tin1 filter has been suggested hy Kennedy and Zellweger (1974) in analyzing for dissolved aluminum and iron concentrations due to the possibility of colloidal metals passing through 0.451~m filters. Dissolved organic carbon sampl,zs were forced through :~ 0.45,urn silver filter using compressed nitrogen gas. Daily precipitation values were obtained from a rain gage at the Blackbird State Forest headquarters, located about 5.5 km east of the study site. One additional shallow groundwater sample, denoted as M-06, was collectea in March 1988 in the upland margin near seasonal Pond 6. The water table was within 3cm of land surl~ce at this site, and the water sample was collected by coring through the thin peat layer and placing the pump inlet hose directly into the standing water in the hole. This sample was collected in order to characterize the groundwater closest to land surface near the ponds. HYDROLOGY

OF GROUNDWATER

AND SEASONAl, PONDS

Three distinct seasonal water-table configurations can be identified at Blackbird State I:'orest. These configurations are related to differences in water-table response to rainfall, location of maximum water-table altitudes, and the presence or absence of standing water in the seasonal ponds. In the following section, each of the three water-table configurations are described and transient water-table response is related to season, changes in water levels in the ponds, and rainfall events. August 1987 to January 1988 water-table comtitions The water-table conliguration from mid-August 1987 to early January 1988 is characterized by a water-table trough beneath the uplands between the seasonal ponds anti little or no surlhce water in the ponds. The water-table profile on 26 October 1987 (Fig, 3) Ibr the north~ south cross-section is typical o1" tile condltiotls between August and January and represents the lowest walel~4able altitudes dtu'ing the 1988 water year. Gradients along the water table were at their highest during this period, with the maximum water-table gradients south of Pond 1. From August through January. local maximum w~ter-table altitudes (subsequently referred to as 'watcrol~ble highs') were located benc~th the iowest etevations~ Hence, the waterotabtc highs during this I'~criodwere located i~l the peats and ct:~y.'; that underlay the seasonal ponds. The minimum water-.table altit~des (subsequently rel;~rred to as "water-table tows') were km:ated beneath the ridge:, frow A,,S~lst through January. Between Pond 1 and Pond 2. fol~ex~mple, the water tab;e sloped downward from the Pond

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r ~ Clay. silt and sand, ]

Sand with some silt and clay Seepage face

Fig. 5. Cruss-seclien showing water-table profiles and pond stages for the northern upland between Pond 2 and Pond I for select dates from 26 October 1987 through I August 1988. Note presence of seepage foees adjacent to Pond I and Pond 2 on 6 May. (Unless otherwise noted, date.- are for 1988. See Fig. 2 for location of section trace. The symbols along the wa!er-lable trace denoic the observed water-table altitude at the well location,)

gages, through the edge and margin wells to the water-table iows at the ridge wells on 26 October 26 (Fig. 5). Similar patterns existed at the southern upland between Ponds 1 and 3 during this period (Fig. 6). Response to recharge varied somewhat with proximity to water-table highs and thickness of the unsaturated zone fro1-.1 Augast through January. The lhicknes8 of Ihe unsaturaled zone in the pond sediments during this period ranged from approximately 9.3 to 0.9 m, and the water table fluctuated more than 0.3 m in response to storms. In contrast, the thickness of the m~saturated zone in the margin and ridge wells ranged from 1.5 to 4.5 m thick. The altitude of the water table at the margin and rMge wells generally increased at ~1steady rate during late 1987, in a manner that appeared unrelated to any ~pecific storm.

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Fig. 6. Cross-section showing water-table profiles and pond stages lbr the southern uphtnd between Pond 1 and Pond 3 for select dates from 26 October 1987 through I August 1988. Note presence of seepage frees adjacent to Pond I on 7 May. (Unless otherwise noted, dates arc tbr 1988, See Fig. 2 for location of section trace. Tile symbols along the water-table trace denote the observed waleHable Mtitude at tile well localion,)

Felwuao; 19,%' to M~O' 19,~'8 u'a/er-table comlitions

From I;ebruary to May 1988, the water-table was generally flat, except during storms when transient water-table mounds formed at the margin wells. This period coincided with the dissipation of the troughs beneath the uplands, the appearance of standing water in the ponds, and the filling of the plmd basins with surface water. By 9 February, ihe waterqable trough loomed in the northern upland was lille(Is and the water levels a| wells R-6 and R+7 were within 12cm of the water levels at M-5 and M-8 (Fig. 5). During this period, the trough beneath the southern upland between Pond 1 and Pond 3 dissipated. By 9 February, the water-table altitudes at wells E-10 and M-04 and the stages of Pond I and Pond 3 were within 3cm of each other (Fig. 61,

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Transient changes in the water-table profiles occurred during storms as transient water-table mounds formed at the margin well sites (Figs. 7 and 8). During dry periods from March through May, the altitude of the water table in the upland between Ponds ! and 2 was 6-18cm higher than that at the ponds (see 4 May water-table profile, Fig. 5). During storms, and for a few days after, transient water-table mounds formed at the margin wells; seepage faces formed at the edges of the ponds and the pond ~;tages incre,qsed incrementally. Tile largest and fastest increases in the stage of Pond 1 are associated with the establishment of transient water-table m~unds (Fig. 7). Similar patterns of water-table mounding occurred at the south edge of Pond 1 during storm events between February and March (Fig. 8), However, between storms, the difference between the water level at well M-4 and Pond 1 was generally less than 5cm. Although gradual increases in pond ,~tages did occur in the absence of a transient mound, most of the increases in the stage of the pond are associated with the presence of a transient mound.

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1988 Fig. 8.1 lydrograph of groundw~ter levels at wells M-04 and E-03 during the period 20 March 1988 through 31 Muy 1988 showing transient response to precipil~ltion ~md ~*ssoeiated increases in ~;tage of Pond I and Pond 3.

In general, the development of transient water-table mounds and associated transient reversal,'~ of gronndwaier flow occurred between wells M-05 and R-06 fl-oln March through May 1988 when ~laily precipitation exceeded 1,5 cm (Fig. 7)~ For example, on 5 M~ly and 6 May, approximately 5 tin of rail1 t~ll, and for Ihe fiqlowing 48 h, the altitude of the water table at Mo05 exceeded tim! tit R-06 by as much as 9cm. During this time period, the difference between the allitude of the water table at M-05 and the stage at Pond 1 increased from 15cm to ~lbout 30cm. A true water-table mound may have appeared on the southern side of Pond 2 between wells M-08 and R-07 during February through M,ly 1988, even though lhe altitude of lhe water iahle ~lf M~08 never exceeded the water table tit R~0? during thi~ peiiod (Fig. 9). Well M~08 is closer in distance and etewtiion fo Pond 2 than either wells M-04 or M-05 are to Pond 1, where transient mounds formed in response to the same storms. Welt M-08 could be downgradient to the north of a transient mound thai forms south of Pond 2. The rapid decay of the transient mound at well M-04 a~d the relatively iarge

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head difference between wells M-04 and R-06 during 21 May also could indicate that well M-04 is on the pond side of the mound that forms south of Pond 1 (Figs. 5 and 7). Hence, as the pond stages and the altitude of the water table in the uplands increase, the crest of the water-table mounds could move away from the ponds. Differences in observed water-table behavior among tt~,e margin wells during February through May could be due to the dynamic nature of the groundwater and st, trace-water interactions. Because the stage,: of the ponds increased throughout Ihis period, the relation between the different transient mounds that tbrmed next to the ponds and the adjacent margin well probably changed over time. Therefore, there is no assurance that the locat_ic~n of each of the margin wells was at the center of the transient mound that formed adjacent to the seasonal ponds. This implies that some of the margin wells could be located oll the pond-side slope of a transient mound. The water levels at the margin wells could be an intermediate indication of the height of the transient moun~l tirol develops at that area,

170

ILL PllII.LIPS AND R J SHEDLOCK

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Fig. 10. H y d r o g r a p h o f g r o u n d w a t e r levels at w e l l s M - 0 5 a n d R - 0 6 d u r i n g t h e p e r i o d I J u n e 1988 t h r o u g h 31 J u l y 1988 s h o w i n g t r a n s i e n t r e s p o n s e to p r e c i p i t a t i o n a n d a s s o c i a t e d i n c r e a s e s in s t a g e o f P o n d I.

June 1988 to August 1988 water-table conditions

The water configuration from Jlme 1988 tl-n'ough August 1988 is characterized by gradual re-establishment of an inverse relation between topography and waterAable conligm'ation, and the presence of standing water in the ponds. As is the case during the period from August 1987 through January 1988, the observed inverse relation between topography and the water-table configuration is the opposite of the common assumption that the water table is a subdued replica of topography. By late May 1988, groundwater and surface-water levels began a steady decline, prestlmably in respo,tse to rising air temperatures and teal'~otltof lhe tbrest canopy in the upland. By I August 1'~88, the stage of surlhce water m the ponds was higher than the water-table altitudes al the margin and ridge wells (Figs, 5,6. and 10). Because the rate of water-table decline at the ridge wells was greater than at the margin wells, by early June 1988, a water-table trough was again established in the northern upland, in gene~ml, the water-table attilude at the ridge and margin wells declined

HYDROI OGY AND CIIEMISTRY OF GROUNDWATER AND SEASONAL PONDS

171

with little interruption during June through August. Light rainfall events do not seem to interrupt this seasonal decline. For example, from 9 Jmie through 16 July, a total of 1.5cm of rain fell, and water levels at wells M-05, R-06 and Pond 1 declined withemt interruption (Fig. 10). However, larger rain~kall events did cause the water table in the margin and ridge wells to rise, although the rise was greater at the margin wells than at the ridge wells. A 5.6 cm rainfall on 17 July and 5.8 cm of rainfall from 21 July to 23 July resulted in a 37 cra rise in the water table at well M-05. In contrast, only a 9cm increase in wate~ level occurred at well R-06 during the same period. A similar pattern of water-table response was also observed at the south edge of Pond 2. On the southern upland, from June through August, the water-table profile slopes fi'om Pond 1 down to Pond 3 (fig. 6). The altitude of the water table at well M-04 decreased to about 43 cm below the stage of Pond 1, and about 3 cm below the stage of Pond 3 by I July. Synoptic water-level measurements showed that the head at well E-10 remained less than the stage of Pond 1 and greater than the altitude of the water table at well M-04. Because the pond stage inundates the sandy sediments near well E-10 through the end of June, it is likely that some surface water infiltra;es to the ~urficial aquifer at the southern end of Pond t during this period. As the surface water disappeared from the seasonal ponds by late August, the water-table configuration was again characterized by an upland trough and dry ponds; therefore, water-table conditions described for August 1987 through January 1988 were re-established for the remainder of the 1988 water year. CHEMISTRY

OF GROUNDWATER

AND

SEASONAL

POND

WATER

The variability in the chemistry of groandwater at Blackbird State Forest is related to topographic position and depth in the aquifer. In general, the water samples collected from the margin sites and surface-.water sites had similar major anion chemistry, pH levels, and aluminum and alkalinity concentrations (Fig. tl and Table 1). In contrast, water-table samples collected at the ridge sites and samples from the deep wells differed from tb,ose collected at the margin wells. The chemical characteristics of the samples co!]ected from the edge wells were transitional between the margin and deep water samples. Although the relations among well type and major water type and concentrations of chei-nical conslituents describcd bclow are based on data collected in March 1988 (2 weeks after a significant rainfall), similar patterns exisl for samples collected at wells E-O3~ M-04, M-05, R-06, R°07, and M-08 in October 1987, and surface-water samples collected at Ponds I and 2 in the

1"/'2

P J, Iq]ILLIPS AND R,J, SIIEDLOCK

CATK

ONS

EXPLANATION S

Seasonal Pond and Sheam

M E

Margin Edge

R

R~dge

D Deep

Fig. I1. Piper diagram showing major ion chemistry fi~r grmmdwater and surface-water samples al Blackbird State Foresl for March t988.

summer of 1987. Relations between hydrology and chemistry are illustrated here using the March 1988 data~ because the greatest number of groundwater and surface-water samples were collected during that period. The Marcia 1988 sampling period also coincided witll the filling of the seasonal ponds. Analysis of samples collected in Marcia 1988 shows that groundwater samples fi'om the margin wells and seven surface-water sites have similar anionic composition, pH values, and alkalinity and aluminum concentrations (Fig. 11 and Table 1). These samples are sulfate-type waters, have decreased pH values (all but one less than 5.1), and decredsed alkalinity concentrations (0-2 mg 1-~). Dissolved alu|ninum concentrations for all bul one of these samples were 150pg I ~ or greater. In -~.:olltrl|~l~the chemistry of tee groundwater samples l¥om the ridge wells differs signilicantly from that of the margin samples. The ridge samples are bicarbonate-type waters, have pH levels 5.5 and above, alkalinity concentrations above 10 mg 1 ~, and dissolved aluminum concentrations of less than

HYI)ROLOGYAn~l)('I{EMtSTRYOF GROt~NI)WA'IE!~AND SEASONALPONDS

173

TABLE 1 Selected water-qualit]/ characteristics for Blackbird State Forest (Water samples collected March 1988; mgl ~, milligrams per liter; #gl -L, micrograms per liter) Site

pH (pH units)

Dissolved ,~rganic carbon (mgl ~)

Sulfate (mgl ~)

52.00 6.30 1.60 4.40

49.00 13.00 20.00 9.90

0 2 2 2

1,800 900 < 10 170

3,500 360 11 1,500

Edge sites E-03 5.15 E-10 5.60 E- 11 5.07

4.70 5.10 3.40

9.6;~ 18.00 16.00

12 18 6

40 30 80

2,800 3,500 380

Ridge sites R-06 5.51 R-07 5.80

1.40 1.40

1.60 1.80

11 ~1

< 10 < 10

11 17

Deep wells 0-05 D-06 D-07 D-09

1.60 6.80 2.60 6.20

9.20 10.00 7.80 10.00

71 34 19 38

< I0 60 < 10 30

81 12,000 4,500 5.000

19.00 30.00 34.00 16.00 14.00 36.00 12.00

23.00 39.00 41.00 29.00 39.00 38.00 54.00

0 1 1 0 I 0 0

530 280 360 150 1~O 50(; 810

620 1,400 670 56 93 520 69

Water-table wells Margin sites M-06 3.90 M-04 4.56 M-05 5.43 M-08 4.61

6.47 5.65 6.02 5.99

Field alkalinity (mgl t as CaCO3 )

Dissolved aluminum (tLgl ~)

Dissolved iron (Itgl i)

Surface water Stream Pond 3 Pond 6 Pond I Pond 8 Pond 7 Pond 2

3.86 5.05 4.93 4.15 4.48 3.70 3.84

10/~g 1 -~. W a t e r s a m p l e s c o l l e c t e d f r o m t h e d e e p w e l l s a r e a l s o b i c a r b o n a t e t y p e waters~ a n d h a v e e l e v a t e d p]-I levels ( g r e a t e r t h a n 5.6), a l u m i n u m c o n c e n t r a t i o n s t h a t r a n g e f r o m less t h a n 10 t o 6 0 t t g 1 ~, a n d a l k a l i n i t y c o n c e n t r a t i o n s o f 19 m g 1-~ o r a b o v e . T h e c h e m i c a l c h a r a c t e r i s t i c s o f t h e s a m p l e s f r o m the t h r e e e d g e wells are g e n e r a l l y i n t e r m e d i a t e b e t w e e n the charac~'erislics

174

P,L PIItLLIP,~ ANI) R J Sltl I'Jl {)CI~

of the margin waters and the deep waters. For example, the anionic compositi~)n of the edge well water samples tends to be a mixture of sulfate and bicarbonate ions (Fig. i 1), The pl-I of the samples collected at the edge wells ranges from 5.1 to 5.6, aluminum concentrations range fi'om 30 to 801tg l i and alkalinity concentrations range from 6 to 18 mg 1 ~. DISCUSSION

These results show that ~,hephenomenon of transient water-table mounding is not restricted to lake settings, but is also an important control on the hydrology of wetland settings. The effects of the transient mounds on the surface-water hydrology are illustrated by the largest increases in pond stages occurring during periods of transient mound formation, The results of the water-table monitoring show that for n~uch of the year, the water table adjacent to, and beneath, the ponds is higher th~l the water table beneath the ridges. Hence, ~s suggested by Winter (i 986), the water-table conligt.ration at this relatively small scale does not mimic surlace topography. The patterns of groundwater and surface-water chemistry found at Blackbird State Forest suggest that transient mounding also affects the chemistry of the surface water. The presence of sulfate-type waters with low pH, high dissolved aluminum, and low alkalinity in the grotmdwater at the pond margins and in the ponds indicates that the short, shallow groundwaterflow paths that discharge to the ponds play a large role in maintaining acidified conditions in the ponds. Previous studies have documented elevated aluminum concentrations in eastern Maryland streams during low-flow periods in the spring (Bachman and Katz, 1986). The results of this study suggest that near-stream transient groundwater mounds are a potential source of the aluminum in these stre,lms+ Shorl grotmdwater-flow paths associated with areas of transient mounding may explain the low pH levels and sldfate-iype waters in lhc margin and pond w~Hers, Puckell (1987) suggested that when sulfate from lbrest throughthll craters the soil ~s a salt or an acid, the pi~i and alkalinity concentrations of the gt'omldwater could be depressed. Puckett predicted that sulfate would remain the predominant anion and alkalinity would remain near zero in water with short groundwater-flow paths, despite elevated carbon dioxide concentrations typically found in soils. Depressed pH levels and calciu~r~-sulfaleqype waters have also been observed in groundwater and surf~cc walers ftca|* wcllalld margins along lh~! so~tthern sho~x-of Lake Michigan, USA (Shed!ock el al., 1988~. i lcnc~:~the ~s:~ociationof sulfate water with wetland margins may be i2~irly widespread. The high tevets of dissolved alun~inum Characteristic of the margin and

IIY|)ROI.O(P,r AND (?13FMISTRY OF (~ROI.INI)WA'I'ER AND SEASONAL IK)NI)S

175

pond-water samples are probably caused by the closeness of the water table to the surface at the margin sites. Bachman and Katz (1986) suggested that elevated levels of dissolved aluminum observed in streams in the Delmarva Peninsula could result from acidified groundwater associated with short, shallow groundwater-flow paths. Thunnan (1985) noted that in the A and B horizons of the soil, dissolved organic carbon can complex aluminum and iron. Hence, the high dissolved aluminum and high dissolved iron concentrations found in most of the margin samples is likely due to the proximity of the water table to the soil horizons. The high aluminum and iron concentrations found for the surface-water samgles is due to the fact that the water table is in the upper soil horizons for most of the year. During periods of transient mounding, the shallow groundwater with high aluminum and iron concentrations flows towards, and fills, the ponds. The applicability of Puckett's and Thurman's results to environments where transient mounds develop is illustrated by the chemistry of site M-06, where the groundwater sample was collected from a peaty soil layer about 5 cm below land surface and less than 9 m from a pond. This site is located in an area where a transient water-table mound forms and, therefore, is characteristic of the groundwater that flows toward seasonal ponds during rainfall. This water sample had the smallest: pH value (3.9), the largest dissolved organic carbon (52 mg 1- ~) concentration, the largest sulfate (49 mg 1-~) concentration, and the largest dissolved al~minum (1800pg 1-I) concentration of any of the groundwater samples. T[~e elevated aluminum and iron concentrations in this sample were likely due to elevated concentrations of dissolved organic carbon, because these metals have a strong affinity for dissolved organic carbon (Thurman, 1985). In genera!, the short flow paths for the margin wells (<60 m) and transient changes in the water-table altitudes suggest low residence time for groundwater at these sites. Apparently, there is not enough time for mineral weathering reactions to produce significant concentrations of alkalinity in this part of the groundwater-flow system. The difference in water chemistry between the shallow grour~dwater at the margin wells and the shallow groundwater at the ridge wells is fikely due to a combination of factors, including differences in depth from the ~oil layer to the water table, length of flow path, and soil organic horizons. Clays in the B horizon of the soil can be expected to retain complexed iron and aluminum, and the concentrations of dissolved organic carbon decrease beneat{~,~the B horizon (Thurman, 1985). Hence, the lower concentrations of aluminum and iron found for the samples collected from the ridge sites are probably related to the greater depth from the soil zone to the water table for the ridge sit,~s in comparison with the margin sites, The thicker unsaturated zone and lack of

176

P,I PItH I.IPS A N D R J St{I I)LO('K

transient water-table mounding at lhe ridge sites may atso result in a longer ~vsidence time for the water in the unsaturated zone, and so greater opportunity for generation of alkalinity and an increase in pH. The soils lit the margin wells also have a thicker organic horizon th!m the soils at tile ridge wells, so that diltErences in soils also conld contribute to the observed differences ill water chemistry. The role that seasonal and event-based changes in the water-table profile play in determining the hydrochemistry of the groundwater from the different topographic zones is further demonstrated by the chemistry of samples collected at the edge wells. During the summer, the edge wells receive seepage fi'om the ponds. The development o!" transient grotmdwater mounds at, or near, the margin wells during the late winter and spring results in a reversal of flow direction in the aquifer beneath the ponds. This pattern of flow reversals in the aquifer beneath the near-shore zone of the ponds is similar to the reversals described by Cherka~ler and Zagvr (1989) in an aquifer beneath the near-shore parl of a lake. Seasonal and event-based reversals in the water-table gradients near the ponds result in the edge wells receiving flow fi'om both the shallow and deep parts of the groundwater-flow system as well as the ponds. This effect seems to be reflected by the transitional nature of the chemistry of groundwater from tile edge wells, The higher pH levels and bicarbonate-type waters characteristic of the samples from near the base of the surficial aquifer illustrates that as groundwater-flow paths lengthen, pH increases and the waters evolve to a bicarbonate water type. Hence, ttle hybrid sulfate-bicarbonate water type characteristic of the edge samples is likely a reflection of mixing of groundwater from the margin, surface-water, and deep sites. CONCI.USIONS These ohm,el'rations ic~ld to Iwo major collclusions about hydrologic invesIigi|liolls of wetland sellings. The first conclusion is thai no single water-table pralilc for a given tinte can adequately ~'epresent the range of groundwaterflow conditions and groundwater interactions with surface water. Tl;c second conclusion is that the hydrology and chemistry of shallow seasonal ponds a re strongly influenced by the adjacent groundwater-flow system, l.arge seasonal and evcnt-hased change~ in ibe watcr+labl~ protilcs were obsm'ved it+ the study al'em Thes{~ watcr4abtc changes arc related io die topography and ttle distance iYom a seasonal pond. Furthermore, the most dynamic part of the water4~|b~e profile is along the upland-wetland margins where transient motmds grow a~d dissipate. Some of these mounds persist for no more than

t I Y D R O | O~JY AND CH| MIS'|RY OF ( ; R O I ] N D W A r E R AND SEASONAl PON|)S

177

a few days. These results also s h o w that the c o m m o n a s s u m p t i o n that g r o u n d water flows from ~:~pographic highs to topographic lows is incorrect in this setting. The chemical similarity o f the waters collected from sites experiencing transient m o u n d i n g an6 the surface waters indicates that transient m o u n d i n g p h e n o m e n a play an importa_nt role in determining the hydrochemistry o f g r o u n d w a t e r and surface water in wetiand settings. The low p H and elevated a l u m i n u m concentrations o~:~served in these waters also suggest that transient m o u n d s can play an i m p o r t a n t role in the acidification o f surface waters. REFERENCES Anderson, M.P. and M unter, J A., 1981. Sc'asonal reversals of groundwater flow around lakes and the relevance to stagnation points and lake budgets. Water Resour. Rcs., 17: I 138-1150. Bachnlan, L.J. and Katz, B.G., 1986. Relationship between precipitation quality, shallow ground-water geochemistry, and dissolved aitxminum in eastern Maryland. Maryland Power Plant Siting Program Report PPSP-AD-14, Maryland Power Plant Sitin~ Commission, Annapolis, MD, 37 pp. Cherkauer, D.S. and Zuger, J.P.. 1989. Ground,~,~ter interaction with a kettle-hole lake: Relation of observations to digital simulations. Jr Hydrol., 109: 167-184. Kenp.cdy, V. C. and Zcllwcgcr, G. W., 1974. Filter pore-size effects on the analysis of AI. Fe, Mn and Ti in water. Water Resour. Rcs., 10: 785-7:~0. Kenoyer, G.J. and Anderson, M.P., 1989. Ground-water',; dynamic role in regulating acidity and chemistry in a precipitation-dominated lake. J. H3,~rol., 109: 287-306. Owens, J.P. and Minard, J.P., 1979. Upper Ccnazoic sediments of the lower Delaware Valley and the north~,~a Delmurva Peninsula, New Jersey, Pennsylvania, Delaware and Maryland. U.S., Geol. Surv., Prof. Pap. 1067-D, U.S. Geological Survey, Washington, DC. 47 pp, Pickett. T. E. and Benson. R. N., 1977. Geology of the Smyrna-Clayton are~l, Delaware, Delaware Geological Survey Map Series, No. 5, I sheet. Puckett, L.J., 1987. The influence of forest canopies on the chemical quality of water and the hydrologic cycle, In: R.C. Avercth R.C. and D.M. McKnight (Editors), Chemical Quality of Water and the HydrokJgic Cycle. Lewis, Chelsea, MI, pp. 3-2L Shedlock, R,J., Loiaeono, N.J. and hllbrigiona, T.E., 1988. Effects of ground water on the hydrochemistry of welhmds at Indiana Dunes, northwest Indiana. In: D. A. Wilcox (Edilor), Interdisciplinary Approaches to Freshwater Wetlands Research. Michigan State University, East Lansing, pp. 37-55. Shedlock, R.J.. Phillips, P.J,. Wilcox, D.A, and Thompson, T.A., 1989. Im¢,ortanee of both regional and locul patterns of flow and chemistry in hydrogeologic investigations of freshwater wetlands. In: 28th lilt. Geological Congress, Abstr., Vol. 3, Was~fington, DC. Skougstad, M.N., Fishman, M.J.. Friedman, L.C., Erdmann. D.E. and Duncan,, S.S., 1979. Methods for determination ofinorganic substances on water and alluvial sedir~ents. U.S. Geological Survey Techniques of Water-Resources Investigations, Book 5, Chapte¢ A I, 626 pp. Thompson, T.A., Miller, C.S.. Doss, P.K.. Thompson. t,,D,P, and Baedke, S.T.. 199~ Landbased vibracoring and vibracore analysis: Tips, tricks and traps. Indiana Geological Survey, Occasional Paper no, 58, I]loomington. IN, t3 pp,

J'7~

P.,I PHII,I ]P~ AND

R.I, SIIEI)I,(E'K

Tburmaa~ E.M. 1985. Organic Geochemistry of Nalaral WaIers. M'irtinus Nijhoff/Dr. W. Junk, Bosto~l, MA, 497 pp. U.S. Department of Agricuhm'e (USDA), 1970, Soil survey of Ncw Castle County, Delaware. L~SI)A, Soil Consc~-vationService, U.S, Department of Agriculture, Washington, DC, 97 pP. Wiuter, T.C., 1983. The inter~etion oJ lakes with variably saturated porous media. Water Resour. Res., 19: I203-1218. Winter, T.C., 1986. Effect of ground-water recharge on configuration of the water table beneath sand dunes and on seepage in lakes in the Sandhills of Nebraska, U.S.A.J. Hydrol, 86: 221-237