ORI GIN AL PA PER

Major Ion Chemistry in a Freshwater Coastal Lagoonfrom Southern Brazil (Mangueira Lagoon): Influenceof Groundwater Inputs

Isaac R. Santos Æ Maria I. Machado Æ Luis F. Niencheski ÆWilliam Burnett Æ Idel B. Milani Æ Carlos F. F. Andrade ÆRichard N. Peterson Æ Jeffrey Chanton Æ Paulo Baisch

Received: 31 August 2007 / Accepted: 4 March 2008 / Published online: 20 March 2008� Springer Science+Business Media B.V. 2008

Abstract This paper characterizes major ion distributions and investigates whether

groundwater exerts a major control on the chemical functioning of Mangueira Lagoon, a

large (90 km long), shallow (*4–5 m deep), and fresh coastal lagoon in southern Brazil.

Water volumes equivalent to *80% of the total annual input are used in the summer for

irrigating nearby rice plantations, the most important regional economic activity. While

Na+ and Cl- are the major ions in local groundwater, Na+ and HCO3- are the most enriched

ions in lagoon water. The ion concentrations measured in Mangueira Lagoon were

homogeneous, except for a few samples affected by rainwater and groundwater inputs. A

shore-normal transect starting at the pump house of a rice irrigation canal indicated strong

groundwater input at this canal. In spite of the small volume contribution (*2% of

precipitation), groundwater discharge accounts for 50–70% of major ion inputs into the

lagoon, with *70% of the groundwater inputs being anthropogenically derived (e.g., from

the rice irrigation canals). This may have serious implications for the management of the

coastal water resources from Mangueira Lagoon and other similar areas as groundwater

associated with agricultural systems may be contaminated by fertilizers and pesticides. The

results imply that groundwater should not be neglected in dissolved species’ budgets even

when its volume contribution is small.

Keywords Submarine groundwater discharge � Biogeochemistry � Permeable sediments �Coastal lagoons � Hydrogeochemistry

I. R. Santos (&) � W. Burnett � R. N. Peterson � J. ChantonDepartment of Oceanography, Florida State University, Tallahassee, FL 32306, USAe-mail: santos@ocean.fsu.edu

M. I. Machado � P. BaischDepartamento de Geociencias, Fundacao Universidade Federal do Rio Grande, CP 474,96201-900 Rio Grande, RS, Brazil

L. F. Niencheski � I. B. Milani � C. F. F. AndradeDepartamento de Quımica, Fundacao Universidade Federal do Rio Grande, CP 474,96201-900 Rio Grande, RS, Brazil

123

Aquat Geochem (2008) 14:133–146DOI 10.1007/s10498-008-9029-0

1 Introduction

Since concentrations of dissolved species in groundwater are often much higher than

surface water, even small inputs of groundwater into lakes, streams, and the coastal ocean

may have important biogeochemical effects. It has been proposed, for example, that high

dissolved nitrogen concentrations in contaminated coastal groundwater may dramatically

change the biogeochemistry of coastal environments within the coming decades (Slomp

and Van Cappellen 2004). Groundwater discharge is usually patchy, diffuse, temporally

variable, and may be in response to multiple driving forces (Burnett et al. 2006). Factors

that can enhance groundwater seepage include high precipitation rates, relief, and per-

meability. In places lacking a well-developed river system, groundwater may exert an even

stronger control on geochemical fluxes (Zektser and Loaiciga 1993).

Many investigations have addressed the effects of point-source pollution into rivers and

lagoons from Brazil and all over the world. However, little is known about the contribution

of diffuse sources, such as groundwater discharge, because it is extremely difficult to

quantify seepage rates and to separate the diffuse groundwater inputs from other processes.

A few recent investigations addressed the input of groundwater into the coastal ocean from

Brazil (Burnett et al. 2008; Godoy et al. 2006; Oliveira et al. 2006; Windom et al. 2006),

but no previous investigations have quantified dissolved species’ inputs via groundwater

into Brazil’s coastal lagoons. They are usually shallow water bodies, oriented shore par-

allel, separated from the ocean by a sandy barrier, hypo- or hypersaline, and highly

productive. Globally, coastal lagoons account for nearly 13% of the world’s coastline and

their basins are among the areas of fastest development in the world (Knoppers et al.

1999).

This paper reports the first hydrochemical observations in groundwaters and surface

waters of Mangueira Lagoon (Fig. 1), a freshwater environment that is part of the largest

coastal lagoon system in the world, the Patos–Mirim–Mangueira system in southern Brazil.

The experimental work was designed to test whether groundwater exerts a major control on

its hydrochemistry. Mangueira Lagoon was selected for this study because it represents an

extreme example of water use control (e.g., rice irrigation) and thus offers a unique

opportunity to investigate the relationships between land use changes and groundwater

discharge. The lack of river input and surface connection to the ocean simplify the

assessment of dissolved species’ sources and may allow a better understanding of the

nearby, more complex lagoons. Most of the southern Brazilian coastal plain is sandy, so

infiltration of industrial and agricultural pollutants into the surficial aquifer may readily

take place wherever inappropriate disposal occurs. The local fertilizer industry has been

considered a potential source of groundwater contamination in the urban areas of Rio

Grande City, located nearly 100 km north of our study site (Mirlean et al. 2005). These

industries release contaminants such as fluorine to the atmosphere. The fluorine is later

precipitated and thus found in high concentrations in shallow groundwater (Mirlean et al.

2002), which in turn may seep into nearby surface water bodies.

2 Materials and Methods

2.1 Study Area

Mangueira Lagoon is a large shallow coastal freshwater body (average 4–5 meters deep;

90 km long) located between an area of intensive rice production and a pristine sandy

134 Aquat Geochem (2008) 14:133–146

123

barrier (Fig. 1). The area of Mangueira Lagoon is nearly 900 km2 with a catchment basin

of comparable size. The regional geology consists of Cenozoic sandy sediments (quartz)

interlayered with former lagoon deposits (Baisch 1994; Beltrame and Tucci 1998; Leao

et al. 1998) accumulated during successive sea-level fluctuations throughout the Quater-

nary (Villwock and Tomazelli 1995). Water inputs into Mangueira Lagoon are through

direct rainfall and groundwater seepage. In spite of the large size of the lagoon, there are no

river inputs. Outputs from Mangueira Lagoon are through evaporation, sub-surface flows,

pumping for irrigation, and seasonal surface flows toward the Taim wetland, which is

located in the northern region of the lagoon and considered a Biosphere Reserve by

UNESCO. The dominance of hydromorphic Pleistocene soils on the west margin of the

lagoon with a low topographic gradient and high water abundance provide excellent

conditions for rice production, the main economic activity in southern Brazil.

Rice irrigation accounts for over 99% of the total water usage in the Mangueira basin.

Lagoon water is pumped to the nearby rice paddies from irrigation canals dredged along the

western margins of the lagoon. Pump houses located at the end of individual canals (Fig. 1)

may deliver up to 15 m3/s of water to a secondary system of canals located at a topographical

level 2–5 m above the lagoon. The water demand for rice irrigation is about 2 l/s/ha,

representing *80% of the annual water input into the lagoon. This diminishes the water

Fig. 1 Map of the study area near the Brazil–Uruguay border showing the location of groundwater(squares) and surface water (circles) samples as well as the Vitor Barbosa (VB) Canal. The dotted anddashed lines represent the 2- and 4-m topographical contours, respectively. The cross at the southwestmargin of the lagoon shows the location of the sediment profile. Mangueira Lagoon waters may flow via theTaim Wetland toward Mirim Lagoon during periods of high water level. The area between Mangueira andMirim Lagoon is used for rice plantations

Aquat Geochem (2008) 14:133–146 135

123

level of Mangueira Lagoon and nearby wetlands (Beltrame and Tucci 1998; Villanueva et al.

2000). The situation is dramatic because the water pumping coincides with the dry season,

when the difference between precipitation and evaporation is -32 mm/month (December),

in contrast to +90 mm/month during the wet season (July). An unknown but presumably large

amount of water runs off the rice fields and returns to the lagoon via surface drainage. The

remainder evaporates or returns as sub-surface flows. In addition to water, the rice mono-

culture uses variable amounts of pesticides, around 180 kg/y/ha of NPK fertilizers, and

*50 kg/y/ha of urea (IRGA 2001; Santos et al. 2004). These contaminants may infiltrate into

the ground and reach the nearby lagoons via groundwater pathways.

In addition to rice production, the fresh nature of its waters, relative high pH (*8), and

elevated content of bicarbonate make Mangueira Lagoon suitable for growing Spirulinaplatensis, an algae with high commercial value (Costa et al. 2002). This has been sug-

gested as an alternative local economic activity, but may be hampered by water use

conflicts with rice farmers and a lack of background information about the lagoon hy-

drogeochemistry. In spite of the ecological and economic value of the lagoon, there are no

previous reports concerning the processes controlling its hydrochemistry.

A companion paper (Santos et al. 2008) estimated groundwater advection rates into the

rice irrigation canals (*20 cm/d) to be 2 orders of magnitude higher than those along the

lagoon shoreline (*0.1 cm/d). Calculated groundwater advection rates were based on a

radon mass balance. No major seasonal changes (wet winter versus dry summer) in

groundwater advection rates were found. This is because the lagoon and the groundwater

levels decrease by similar amounts (*80 cm) during the dry season, keeping the hydraulic

gradient relatively constant. A canal at the northern end of the lagoon, where the topo-

graphical gradients are less steep (see contours in Fig. 1), had lower advection rates than

canals in the south. In spite of the relatively small area of the canals (a total of 36 canals

covering 0.2 km2), they contributed nearly 70% of the total (*57,000 m3/d) groundwater

input into Mangueira Lagoon. While these groundwater fluxes represent only *2% of the

average annual precipitation flux (1212 mm or 1.1 9 109 m3 for the total area of the

lagoon), they are biogeochemically significant as dissolved species’ concentrations in

groundwater are much higher than in rainwater.

2.2 Sampling and Analysis

Mangueira Lagoon and associated groundwater sampling was carried out in August 2006. A

total of 11 groundwater samples were collected from wells ranging from 3 to 11 m deep. Prior

to sampling, the wells were purged long enough to replace their volumes at least 3 times.

Sampling also included 27 Mangueira Lagoon water samples along a north–south transect

(both west and east margins) and 7 samples along a transect from the pump house of a rice

irrigation canal (Vitor Barbosa Canal) to 1,000 m offshore (Fig. 1). Not surprisingly, no

temperature and conductivity changes were observed between the bottom and surface waters

of the shallow lagoon. Therefore, all the samples were collected from the surface.

Samples were filtered with membrane filters (0.45 lm Millipore�) immediately after

collection and kept on ice in acid-clean plastic vials until analysis. Conductivity, pH, and

temperature were determined in situ with portable YSI electrodes. Major cation and anion

analyses were performed by ionic chromatography (Methrom�). Anions were separated

using a Metrosep A Supp5-100, 4.0 9 100 mm column and a NaHCO3–Na2CO3 mobile

phase at 0.6 ml min-1 flow rate. Cations were collected using a 4–mmol l-1 tartaric acid

and a 0.75-mmol l-1 dipicolinic acid solutions in a Metrosep C2-100 column at a

136 Aquat Geochem (2008) 14:133–146

123

1 ml min-1 flow rate (Mirlean et al. 2005). Sample dilution was conducted whenever

necessary with Milli-Q water. For determining total alkalinity, the samples were titrated

(Titrino 702SM-Methrom�) to pH 4 with hydrochloric acid (Chen et al. 1996). Alkalinity

(ALK) was defined as the equivalent sum of the bases that are titratable with strong acid,

representing the acid neutralizing capacity of the aqueous system (Millero 1996).

A soil profile was sampled (hand-augered) near Vitor Barbosa Canal (Fig. 1) to acquire

a basic understanding of the local hydrogeological setting as no background information

about the aquifer material could be found in the literature or with local authorities. Basic

grain size and loss on ignition analyses were determined via wet-sieving and by igniting

the samples at 450�C for 24 h, respectively. Slug tests representing the first estimates of

regional hydraulic conductivities were conducted in monitoring wells using automatic

sensors (CTD Divers Van Essen�) following recommendations described elsewhere (Fetter

2001).

3 Results and Discussion

3.1 Ion Distributions

A balance separately summing the anions and cations in terms of equivalents per liter was

employed to check the accuracy of chemical analyses (Fig. 2). From a total of 45 samples,

the error was less than 10% for 29 samples (66%), between 10 and 20% for 9 samples

(20%), and greater than 20% for only 6 samples (14% of the total), indicating an overall

good accuracy of analysis as compared to other investigations of major ions in natural

waters (Campos et al. 1998; Fritz 1994). Part of the uncertainty may be explained by the

presence of other naturally occurring ions present in low concentrations, including but not

limited to ammonium, nitrate, phosphate, strontium, and boron. Nitrate, ammonium, and

phosphate concentrations were either non-detectable (the ionic chromatography system

was not optimized for such components) or orders of magnitude below the major ions.

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14 16 18OH- + F- + Cl- + SO4

2- + HCO3- (meq)

H+ +

Na+ +

K+ +

Ca2+

+ M

g2+

(meq

)

Fig. 2 Scatter plot of the sum ofanions versus the sum of cationsin Mangueira Lagoon (blackcircles) and adjacent groundwater(open squares). The linerepresents a slope of 1

Aquat Geochem (2008) 14:133–146 137

123

Hence, the agreement found for most samples indicates a high analytical accuracy and

suggests that all the ionic species present at significant concentrations were identified.

Averages and standard deviations of dissolved chemical species in Mangueira Lagoon

water, adjacent groundwater, and rainwater (Viana 2005) are presented in Table 1. The

average ionic strength was 0.023 and 0.003 for groundwater and surface water, respectively.

Even though Mangueira basin groundwater is probably recharged by regional precipitation,

the higher ion concentrations in groundwater indicated that as water flows through the

aquifer, it assumes a diagnostic chemical composition dependent upon the residence time,

physico-chemical conditions, interactions with soil particles, and possible anthropogenic

sources. The major ion composition of Mangueira Lagoon is relatively constant, with spatial

variations less than 32%. Groundwater concentrations, in turn, are much higher and more

variable with relative standard deviations ranging from 36% (Na+) to 81% (K+). For the pH

observed in Mangueira Lagoon (*8) and its adjacent groundwater (*6–7), equilibrium

HCO3- concentrations are at least one order of magnitude higher than CO3

2- (Millero 1996), so

alkalinity was assumed to be equivalent to HCO3- concentrations.

In spite of the small overall variability, lower ion concentrations (except F-) occurred in

samples 1 through 6, collected in the southern end of the lagoon (Fig. 3). The month prior

to sampling in August 2006 was extremely rainy compared to the historical average of

104 mm (Beltrame and Tucci, 1998). While total precipitation was officially measured to

Table 1 Average (and standard deviation) concentrations (in mM, except conductivity in lS/cm and Eh inmV) in surface water, groundwater, and rainwater (from Viana 2005) and determined annual fluxes ofgroundwater- and rainwater-derived chemical constituents into Mangueira Lagoon

Temp Cond pH Eh F- Cl- SO42- HCO3

- Na+ K+ Ca2+ Mg2+

Concentrations (mM)

Groundwater

Av. 16.7 785 6.5 111 0.003 6.7 0.314 2.29 7.5 0.032 0.931 0.441

St Dev. 1.8 493 0.4 66 0.003 2.6 0.164 1.06 2.6 0.036 0.701 0.120

Surface water

Av. 13.7 208 7.8 166 0.003 1.3 0.081 1.56 1.4 0.063 0.490 0.139

St Dev. 1.2 40 0.4 21 0.001 0.3 0.020 0.15 0.3 0.020 0.099 0.025

Rainwater

Av. – 5.4 – 0.006 0.1 0.003 0.03 0.1 0.012 0.005 0.008

Fluxes (in 106 mol/yr)

Groundwater

Av. 0.07 140 6.5 47.6 156 0.7 19.4 9.2

St Dev. 0.06 55 3.4 22.1 55 0.8 14.6 2.5

Rainwater

Av. 6.97 143 3.7 27.5 85 12.9 5.8 8.3

Ratios groundwater:rainwater

Concentrations 0.5 51.9 92.8 91.5 96.8 2.7 175.4 58.4

Fluxes 0.0 1.0 1.8 1.7 1.8 0.1 3.3 1.1

Relative contribution (%)

Groundwater 1 50 64 63 65 5 77 52

Rainwater 99 50 36 37 35 95 23 48

138 Aquat Geochem (2008) 14:133–146

123

be 144 mm in Rio Grande, local farmers reported up to 300 mm in the southern end of the

lagoon during this period. Therefore, despite limited observations for such a large area,

the intense rainfall preceding the sampling was not spatially uniform, perhaps explaining

the relatively lower ion concentrations in the southern end of the lagoon. The influence of

precipitation acting to dilute lagoon water in the south is also indicated by lower pH values

and higher fluoride concentrations (Fig. 3), since fluoride is relatively enriched in rainwater

(Table 1).

A reduction in pH values in the area affected by rainfall indicates that the carbonate

system is not well buffered in Mangueira Lagoon. Calcite precipitation buffers pH and

maintains it in a slightly alkaline condition. Precipitation of calcite may occur from waters

where initial alkalinity exceeds Ca2+ concentration, resulting in Ca2+ being progressively

removed from solution (Banks et al. 2004). Calcite saturation in the presence of significant

alkalinity prevents accumulation of Ca2+ in the waters, and thus also prevents gypsum

saturation from being achieved, allowing SO42- to accumulate in the water. Calcium and

alkalinity concentrations vary with conductivity in groundwater (Table 2), suggesting that

calcite saturation and precipitation is not limiting Ca2+ and HCO3- accumulation (Banks

et al. 2004).

The high alkalinity and pH is a unique characteristic of Mangueira Lagoon compared to

the other coastal lagoons of southern Brazil. Alkalinity may control the plant composition

and distribution in aquatic environments (Radke et al. 2002; Vestergaard and Sand-Jensen

2000), perhaps differentiating Mangueira Lagoon’s plant communities from other regional

water bodies. In addition to groundwater inputs, the equilibration of the carbonate-rich

sediments with the overlying water may explain the high Ca2+ and HCO3- in Mangueira

Lagoon waters (Pillsbury and Byrne 2007). The use of CaCO3 to increase the pH of the rice

field soils (IRGA 2001) is another possible (but likely minor) source of these ions to

regional waters. The carbonate content in Mangueira Lagoon sediment ranges from 3 to

27% (authors’ unpublished data), in contrast to the relatively carbonate-poor sediments of

the other nearby lagoons, such as Patos and Mirim Lagoons (Calliari 1980).

Fig. 3 Distribution of selected variables in Mangueira Lagoon surface waters. Darker colors representhigher values. Ion concentrations in mM

Aquat Geochem (2008) 14:133–146 139

123

While the north–south transect indicated the influence of precipitation diluting lagoon

water over large scales (tens of kilometers), the shore-normal transect showed steep

hydrochemical gradients (Fig. 4) over small spatial scales (\100 m). The highest ion

concentrations were found closest to shore (by the pump house of the rice irrigation canal),

where 222Rn, a reliable groundwater tracer (Burnett and Dulaiova 2006), was also highest.

Significant correlations between conductivity and most ions (Table 2; Fig. 4), indicate that

conductivity can be used to identify the areas of high ion concentrations and high

groundwater discharge.

3.2 Hydrochemical Facies

To describe the hydrochemical facies of Mangueira Lagoon, the ionic species were plotted

on a Piper diagram, on the basis of the milliequivalent percentages of each cation or anion

(Fig. 5). The Piper diagram shows that the chemical composition of groundwater is dif-

ferent than that of surface water. While Na+ and Cl- are the major ions in groundwater,

Na+ and HCO3- are the most enriched ions in lagoon water. There is a mixing gradient

between groundwater and lagoon water samples, which can be seen clearly on the two

ternary plots of Fig. 5. This mixing is highly accentuated in the shore-normal transect

samples (Figs. 4, 5), indicating strong interactions between the aquifer and surface waters

Table 2 Pearson correlation coefficients for the variables under investigation. Bold values are significant atp \ 0.05

Cond pH Eh ALK F- Cl- SO42- Na+ K+ Ca2+

Groundwater samples (n=11)

pH 0.253 1.000

Eh -0.075 -0.096 1.000

ALK 0.647 0.438 -0.532 1.000

F- 0.701 0.748 -0.195 0.629 1.000

Cl- 0.975 0.701 -0.216 0.615 0.632 1.000

SO42- 0.333 0.166 0.517 0.281 0.641 0.295 1.000

Na+ 0.739 0.455 0.159 0.448 0.712 0.776 0.646 1.000

K+ 0.471 0.507 0.414 0.172 -0.339 0.325 0.141 -0.087 1.000

Ca2+ 0.719 0.876 -0.099 0.264 0.393 0.665 0.014 0.345 0.682 1.000

Mg2+ 0.953 0.732 -0.061 0.580 0.666 0.929 0.382 0.698 0.655 0.716

Mangueira Lagoon samples (n = 27)

pH 0.791 1.000

Eh -0.691 -0.600 1.000

ALK 0.311 0.452 -0.136 1.000

F- -0.056 -0.111 0.081 -0.013 1.000

Cl- 0.766 0.759 -0.678 0.178 0.133 1.000

SO42- 0.574 0.644 -0.620 0.251 -0.182 0.657 1.000

Na+ 0.737 0.725 -0.690 0.120 0.133 0.993 0.646 1.000

K+ 0.331 0.141 -0.453 -0.127 -0.319 0.435 0.274 0.454 1.000

Ca2+ 0.806 0.859 -0.744 0.339 0.110 0.844 0.678 0.822 0.342 1.000

Mg2+ 0.530 0.324 -0.635 -0.120 0.013 0.677 0.511 0.709 0.613 0.538

140 Aquat Geochem (2008) 14:133–146

123

in the irrigation canals and a progressive dominance of Cl- over HCO3- as the groundwater

source is approached.

The dominance of Na+ and Cl- in groundwater has also been found in Rio Grande,

where the rainwater infiltration associated with the mineralogical composition of sediments

(essentially quartz) generates slightly acidic (pH 5.5) groundwater containing low con-

centrations of Ca2+ and Mg2+ and higher amounts of Na+ and Cl- (Mirlean et al. 2005). In

the Guarani aquifer, which covers most of the continental area of southern Brazil and is

composed mainly of siliceous and carbonaceous rocks, HCO3- greatly dominates the water

composition in terms of dissolved anions, whereas Na+ followed by Ca2+ dominates the

cations (Bonotto 2006). The very low K+ concentrations in Mangueira Lagoon ground-

water are probably a consequence of the dominance of sands (quartz) and low content of

K-feldspars associated with low potassium mobility.

3.3 Groundwater-Derived Ion Fluxes

Groundwater-derived ion fluxes can be calculated by multiplying the average ion con-

centration in groundwater by the total groundwater discharge rate into Mangueira Lagoon,

which was conservatively estimated to be 57,000 m3/day from a 222Rn mass balance

(Santos et al. 2008). The groundwater-derived ion fluxes ranged from 0.07 to 156 9

106 mol yr-1 for F- and Na+, respectively (Table 1). By repeating the same procedure

using the average annual precipitation (1,212 mm or 1.1 9 109 m3 for the total area of the

lagoon) and the average ion concentration in the marine aerosol-enriched regional pre-

cipitation (Viana 2005), one can compare the relative contributions of groundwater and

rainwater, the two external sources of water to Mangueira Lagoon. These results indicate

that in spite of the small volume contribution, groundwater-derived ion fluxes contribute a

0 100 200 300 400 800 1000

pH

7.0

7.2

7.4

7.6

7.8

8.0

0 100 200 300 400 800 1000M

g2+

(m

M)

0.10

0.15

0.20

0.25

0.30

0 100 200 300 400 800 1000

Ca2+

(m

M)

0.0

0.3

0.6

0.9

1.2

1.5

0 100 200 300 400 800 1000

SO

42- (m

M)

0.03

0.04

0.05

0.06

0.07

0.08

0 100 200 300 400 800 1000

Cl- (

mM

)

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 100 200 300 400 800 1000

Na+

(m

M)

1.2

1.6

2.0

2.4

2.8

0 100 200 300 400 800 1000

K+ (

mM

)

0.00

0.02

0.04

0.06

0.08

0.10

0 100 200 300 400 800 1000

222 R

n (d

pm/L

)

0

10

20

30

40

50

0 100 200 300 400 800 1000

HC

O3-

(mM

)

1.0

1.5

2.0

2.5

3.0

3.5

Distance from pump house (m) Distance from pump house (m)Distance from pump house (m)

Fig. 4 Results of shore-normal transect off Vitor Barbosa Canal. Data for 222Rn from Santos et al. (2008)

Aquat Geochem (2008) 14:133–146 141

123

large amount of dissolved species into the lagoon. The relative groundwater contribution

ranged from 1% of total F- (which is highly enriched in regional precipitation due to

anthropogenic inputs from nearby Rio Grande) to 77% for total Ca2+ input (Table 1).

An alternative way to check whether the fluxes presented in Table 1 are correct is to use

elemental ratios in the different endmembers to derive the relative contribution of the water

sources. Changes in ion ratios can be used to assess diagenetic reactions, biological pro-

cesses, and mixing between water sources (McGowan and Martin 2007). The high ion

concentrations in the lagoon may reflect not only inputs from groundwater and deposition

of marine aerosols, but also concentration by evaporation and various biogeochemical

reactions. For example, dissolution or precipitation of carbonate may affect Ca2+, HCO3-,

and even SO42- cycling in Mangueira Lagoon. As Cl- and Na+ are conservative and equally

affected by evaporation (Radke et al. 2002), changes in their ratios can be used to identify

sources. Therefore, by assuming that the system is in steady state and that the observed

average concentrations (Table 1) are representative of the endmember values, a simple two

endmember mixing model was applied to explain the relative contribution of Cl-/Na+

sources:

fRW þ fGW ¼ 1 ð1ÞfGWClGW þ fRWClRW

fGWNaGW þ fRWNaRW

¼ ClML

NaML

ð2Þ

Fig. 5 Piper diagram for Mangueira Lagoon water, adjacent groundwater, VB shore-normal transect, andyear-round average rainwater. Rainwater composition was obtained from Viana (2005) for a site *100 kmnorth of Mangueira Lagoon

142 Aquat Geochem (2008) 14:133–146

123

where fGW is the fraction of groundwater and fRW is the fraction of rainwater contributing

to the Cl- and Na+ content of Mangueira Lagoon (ML) water. The average molar Cl-/Na+

ratio in Mangueira Lagoon water was 0.98 and in the associated groundwater was 0.90. By

assuming that regional precipitation has a Cl-/Na+ ratio similar to that of seawater (1.15),

we found that groundwater contributes *70% of the total Cl- and Na+ to Mangueira

Lagoon. If we assume that the precipitation Cl-/Na+ endmember is comparable to that

observed for regional precipitation in Rio Grande (1.67—Viana 2005), the groundwater

contribution would be as high as 90%.

In spite of the inherent limitations of this simple approach, the values derived from this

mixing model indicate that groundwater is a major source of dissolved species into

Mangueira Lagoon, and agrees reasonably well with the values derived from the flux

calculations shown in Table 1. This supports the hypothesis that, even though groundwater

contributes \2% of the total water input, it plays a significant role in Mangueira Lagoon

biogeochemistry.

Nearly 70% of the total groundwater inputs (Santos et al. 2008) are associated with

anthropogenic alteration of the hydrologic regime. The dredging of the irrigation canals

has apparently removed aquitards which previously restricted upward advection from the

underlying permeable strata. A preliminary characterization of the local hydrogeology

(e.g., grain size, organic matter content, soil description, and hydraulic conductivity) is

shown in Fig. 6. The upper 2-m layer is impermeable and is composed of a topsoil layer

(the uppermost layer of soil had the highest concentration of organic matter and contained

the finest-grained sediments). This allows the development of a perched aquifer when the

0 5 10 15

Organic Matter (%)

0

1

2

3

4

5

6

7

8

9

10

40 60 80 100

Sand (%)

Dep

th (

m)

Topsoil

Impermeable mud

Muddy-sand

Fe-coatedsand

Green sand

Yellow sand, mud pellets

Yellow sand, biodetritus

Yellow sand

K (m/d) Sediment Description

8

12

Fig. 6 Sand, organic matter, hydraulic conductivity (K), and description of a vertical sediment profilecollected in a rice field located on the west coast of Mangueira Lagoon. The vertical line on the K columnindicates the length and vertical location of the well screen

Aquat Geochem (2008) 14:133–146 143

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rice fields are flooded (Santos et al. 2008). The deeper layers, however, are highly per-

meable sands (hydraulic conductivity reaching *12 m d-1). Direct exposure of the highly

permeable underlying layers in the canals allows groundwater to readily flow into the

lagoon. Thus, the lagoon may have been experiencing changes in its hydrochemistry since

the canals were dredged in the 1960s and 1970s. It is likely that before the 1960s, dissolved

species’ inputs into Mangueira Lagoon were derived almost exclusively from precipitation,

whereas today, groundwater exerts a stronger control on Mangueira Lagoon

hydrochemistry.

The groundwater contribution would likely be even more important for solutes asso-

ciated with fertilizer and pesticide use in the rice fields rather than solutes enriched in

coastal aerosols, such as the ones investigated here. Even though the surface soils on the

western margin of the lagoon where the rice fields are located appear to be impermeable

and considering that rice takes up most of the dissolved nutrients (Diel et al. 2007), water

enriched in contaminants may infiltrate into the surface aquifer in specific areas, such as

the upper reaches of the irrigation canals, and later be discharged into the lagoon. The

heavy use of pesticides and fertilizers that are highly enriched in toxic metals (Friedrich

et al. 2006; IRGA 2001; Mirlean et al. 2003), suggests additional investigations examining

the importance of groundwater in transporting dissolved species into Mangueira Lagoon.

4 Conclusions

The ion compositions of Mangueira Lagoon and adjacent groundwater were investigated to

assess the role of groundwater discharge in the lagoon’s hydrochemistry. While Na+ and

Cl- are the major ions in groundwater, Na+ and HCO3- are the most enriched ions in

surface water. HCO3- enrichment in the lagoon is probably associated with mineral dis-

solution from carbonate-rich bottom sediments. Significantly higher ion concentrations

near the pump house of a rice irrigation canal were consistent with a nearshore ground-

water source. Groundwater accounted for 50–70% of the total Na+, Cl-, and most other

ions’ input into Mangueira Lagoon, where *70% of total groundwater inputs occur in the

irrigation canals. These canals may therefore represent an important source of other dis-

solved chemical species enriched in groundwater and thus should be considered a priority

area for future investigations in Mangueira Lagoon. This may have serious implications for

the management of coastal lagoons from southern Brazil and other similar areas as

groundwater associated with agricultural systems may be contaminated by fertilizers and

pesticides. The results imply that groundwater should not be neglected in dissolved spe-

cies’ budgets even when its volume contribution is small.

Acknowledgments I. Santos holds a CAPES/Fulbright fellowship (2150/04-2). This project was spon-sored by CNPq (552715/2005-0, 301219/2003-6, and 305375/2006-7) and FAPERGS (0518017) grants toL. Niencheski and a NSF (OCE05-20723) grant to W. Burnett. Many thanks to Vanderlen Miranda andTamaragiba Pereira for assistance in the field and Neusa Teixeira for performing grain size analysis. We areindebted to the many local residents (especially Mrs. Aldo Giudice, Isaac M. Rodrigues, and Amauri Senna)whose help and hospitality allowed the execution of this project.

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