Journal of Water Resource and Protection, 2013, 5, 792-800
http://dx.doi.org/10.4236/jwarp.2013.58080 Published Online August 2013 (http://www.scirp.org/journal/jwarp)
Assessment of Acid Deposition Effects on Water Quality of
the Upper Rio Grande River Section in Texas
Qin Qian1*, Badri Parajuli1, Qi Fu2, Kaiming Yan1, John L. Gossage3, Thomas Ho3
1Department of Civil Engineering, Lamar University, Beaumont, USA
2Department of Earth and Atmospheric Sciences, University of Houston, Houston, USA
3Department of Chemical Engineering, Lamar University, Beaumont, USA
Received June 4, 2013; revised July 5, 2013; accepted August 6, 2013
Copyright © 2013 Qin Qian et al. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Airborne pollutants such as and
O that cause acid rain may pollute water resources via acid deposition.
However, such effects on the water quality of the upper Rio Grande River section in Texas have not been systematically
studied. The objective of this study is to collect and analyze field data, and perform hydrological and water chemistry
analyses to assess acid deposition effects on the river water quality. The analysis of the precipitation data indicates that
the concentrations of ions decrease as the quantity of precipitation increases. The precipitation with higher concentra-
tions of and
O has a lower pH while that with higher concentrations of Ca2+ and Na+ has a relatively higher
pH value. The analysis of river data demonstrates that the pH value, Dissolved Oxygen (DO), and Total Dissolved Solid
(TDS) generally decrease when the flow rate increases immediately following precipitation events. The drop in pH fol-
lowing a precipitation event is due to the low pH in the precipitation. The DO and TDS decrease after the precipitation
due to the increased flow rate. The slightly higher pH and lower DO values in the eastern section of the river (where the
basin is limestone-dominated) as compared to the western section is due to the limestone erosion caused by the acid
deposition. The annual stone loss by the acid deposition is about 72,000 m3. The fluctuation between the pH value and
the temperature suggests the effect of CaCO3 solubility on the pH value. The water chemistry analysis using Geochem-
ist’s Work Bench (GWB) has been performed to estimate the effect on the oscillation of CaCO3 dissolution-precipita-
tion process. The equilibrium pH decreases with decreasing temperature, but increases as the CaCO3 concentration de-
creases. The effect of limestone on observed daily pH fluctuations appears to be supported by the simulation.
Keywords: Acid Deposition; Precipitation; Rio Grande River; Water Quality
Acid deposition, commonly known as acid rain, is one of
the important culprits for water quality and ecology sys-
tem degradation in the USA [1-3]. Acid deposition is
formed from airborne particulate pollutants, such as SO2
and NOx, which react with water, oxygen, carbon dioxide,
and sunlight in the atmosphere to result in sulfuric
(H2SO4) and nitric (HNO3) acids, the primary agents of
acid deposition. Airborne pollutants are transported into
the river basin along with the air mass and deposit in the
basin as dry and wet deposition . Before getting into
the water body, acid deposition within the catchment
flows through forests, fields, buildings, and roads, mixes
with impurities and undergoes a series of chemical reac-
tions on its course. In areas where buffering capacity is
low, acid rain releases aluminum from soils into lakes
and streams, which is highly toxic to many species of
aquatic organisms . Acidity of the water body affects
the aquatic life inhabiting it since the water pH deter-
mines the solubility and biological availability of chemi-
cal constituents such as nutrients and heavy metals .
Solubility of many metals increases at lower pH resulting
in the increasing toxicity . Water with excessively
high pH causes a bitter taste, encrusts water pipes and
water-using appliances with deposits, and depresses the
effectiveness of chlorine disinfection, thereby causing the
need for additional chlorine . Only a limited pH range
between 6.5 and 8.5 of water is useful for drinking, irri-
gation, industrial use if other water quality parameters
are within the normal values and this pH range is typical
of most major drainage basins of the world .
Data provided by the National Atmospheric Deposi-
opyright © 2013 SciRes. JWARP
Q. QIAN ET AL. 793
tion Program (NADP) showed more acid rain events in
the Rio Grande River basin in recent years. With in-
creasing population and urbanization in the Rio Grande
basin, the recent Texas Commission on Environmental
Quality (TCEQ) studies have reported high concentra-
tions of sulfates and nitrates in some segments of the
Upper Rio Grande River in Texas. The results from
Community Multi-scale Air Quality (CMAQ) model
version 4.6  indicate that the deposition of pollutants,
sulfate in particular, can be a serious problem on water
sustainability in the Rio Grande River in Texas . The
major sources of airborne particulate pollution that pose
risks to the sustainable use of Rio Grande River water
may be vehicular activities and industrial activities (re-
fineries) from Mexico and/or the Texas Gulf Coast re-
gion . The acid deposition attaches to the beautifully
etched limestone cliffs in the Sierra del Caballo Muerto
and in Big Bend’s canyons to dissolve a tiny bit of calcite,
which is transported away by rapid runoff and flash-flood-
ing following summer thunderstorms . Such impact
caused by the acid deposition in streams is a complex
physical and chemical process .
The objective of this study is to assess the impact of
acid deposition on the water quality in the Rio Grande
River section between El Paso and the Amistad Reser-
voir in Texas. It is accomplished through collecting and
analyzing field measurement data and performing hy-
drological and water chemical analyses on river water.
The field data collected and analyzed include the amount
of precipitation and the corresponding water chemicals in
the precipitation, including pH and concentrations of
O, Ca2+, Na+, and other species, as well as the
corresponding water pH, total dissolved solid (TDS), and
Dissolved Oxygen (DO) in the river water after precipi-
tation events. The hydrological analysis demonstrates the
relationship between flow rates and precipitations, in turn
to evaluate the acid deposition impact on the river water
quality. To assess the limestone erosion due to acid de-
position, the total stone loss has been estimated. A geo-
chemical analysis by Geochemist’s Work Bench (GWB)
has been performed to estimate the effect of the oscilla-
tion of calcite dissolution-precipitation process on the
equilibrium pH in the river.
2. Study Region
The Rio Grande (Figure 1) is the fifth longest river sys-
tem in North America. It runs 1800 miles from its origin
in the southern Colorado Rocky Mountains before it
drains into the Gulf of Mexico. The Upper Rio Grande
River Section in Texas between El Paso and the Amistad
reservoir serves as the border between Mexico and Texas.
It is arid to semi-arid climate desert ecosystems. It en-
compasses a total of 51,475 square miles of which 16,845
square miles are upstream of the Rio Conchos and 34,630
Figure 1. The geography map of the Rio Grande River Ba-
are downstream of the Rio Conchos . The three ma-
jor tributaries are the Rio Conchos from Mexico with a
watershed area of 26,404 mi2, the Pecos River with a wa-
tershed area of 35,308 mi2, and the Devils River with a
watershed area of 4305 mi2 . The watershed of Pecos
River and the Devils River are located in Texas and sup-
ply the water to the Amistad reservoir. The first reach
between El Paso and Presidio, upstream of the Rio Con-
chos, contains about 55% alluvial tar, 40% desert moun-
tain terrain, and 5% aquifer recharge zone. The second
reach between Presidio and Amistad Dam, below the Rio
Conchos, contains 40% Desert Mountain and canyon
land (volcanic rock), 55% massive limestone, and 5%
chalk . The Rio Conchos enters the Rio Grande near
Presidio, Texas, just upstream of Big Bend National Park
and Ojinaga, Mexico, in a region of mountains and can-
yons. Along the entire river, water lost through evapora-
tion exceeds water gained from precipitation. Because of
the dry, desiccated state in the Rio Grande watershed,
most rainfall events are absorbed by the watershed with-
out creating any appreciable runoff. By some accounts,
less than four percent of the precipitation that falls on
this watershed reaches the Rio Grande .
3. Analysis of Field Measurement Data
3.1. Characteristics of Wet Deposition
The wet deposition data in the Rio Grande River basin
Copyright © 2013 SciRes. JWARP
Q. QIAN ET AL.
from 1994 to 2011 at the Big Bend National Park (TX04,
Brewster County), Sonora (TX16, Edwards County), and
Guadalupe Mountains National Park Frijole Ranger Sta-
tion (TX22, Culberson County) were collected from the
National Atmospheric Deposition Program (NADP).
Sample handling procedures at all NADP sites were
changed substantially on 01/11/1994 to reduce contami-
nation from the sample shipping container. Therefore, the
annual precipitation quantity and pH as well as deposi-
tion data (kg/ha) of nitrate, sulfate, ammonium, calcium,
magnesium, potassium, sodium, and chloride ions after
01/11/1994 were retrieved. The average annual precipita-
tion of these three stations is 42.6 cm/yr. The maximum
quantity (70.9 cm/yr) occurred in 2004 and the minimum
quantity (25.4 cm/yr) occurred in 2011. The annual pre-
cipitation and the number of raining day have been plot-
ted in Figure 2 (no daily data recorded after 05/08/2007
in 2007). It shows TX16 is the wettest station, while the
TX04 is the driest station. However, TX22 has more
raining days than TX16. The monthly precipitation
(Figure 3) shows higher precipitation quantity occurs in
summer for all three stations. The maximum daily pre-
cipitation at each station occurred on 08/17/2006 at TX
04, 10/09/2011 at TX16 and 10/7/2003 at TX22. The
average, maximum and minimum field pH of annual pre-
cipitations is 5.24, 5.41 and 5.03 respectively, which in-
dicates the basin is experiencing acid deposition.
The annual deposition mass was calculated graphically
by multiplying the deposition by the corresponding drai-
nage area. Figure 4 shows the maximum depositions of
sulfate (5267 tons) and nitrate (4227 tons) occurred in
2001, the maximum depositions of ammonium (1678
tons) and calcium (3688 tons) occurred in 2011, and the
maximum quantities of chloride (1141 tons), sodium
(845 tons), magnesium (249 tons) and potassium (135
tons) were deposited in 1996. In addition, for each of
these eight species the annual deposition in 2011 was
higher than in any other year from 2003 to 2010 despite
the fact that the precipitation quantity is the lowest, i.e.
the drought condition may cause acid pulse because sul-
phur dioxide (SO2) deposited onto the soil is reduced to
sulphur and oxygen; then this is re-oxidised in combina-
tion with runoff to form acids in the soil or discharge to
the river . The newest data in 2012 indicate the
drought in the basin is continuing.
To illustrate the precipitation chemistry, the statistical
analysis was applied to the weekly weighted mean wet
deposition and weekly precipitation quantity data at the
three stations. The different ion concentrations of a typi-
cal acid rain (pH < 5.6) and normal rain (pH > 5.6) is
graphed in Figure 5. It indicates the acid rain has higher
concentrations of anions such as sulfate and nitrate but
the normal rain has higher concentrations of cations such
as calcium and sodium. With the weekly precipitation
Figure 2. The annual precipitation and the number of rain-
ing day at three stations.
Figure 3. The monthly precipitation at the three stations.
data at each station between 1994 and 2011, the number
of weekly precipitation events was counted for every 10
mm interval. There are more than 200 events at each sta-
tion with less than 10 mm quantity and the number of
events decreases with increasing precipitation quantity as
shown in Figure 6. The weekly weighted ion concentra-
tions of ammonium, calcium, chloride, magnesium, ni-
trate, potassium, sodium and sulfate at 95% confidence
interval were plotted against the 10 mm interval weekly
precipitations. The result, that less than 10 mm weekly
precipitations have much higher ion concentrations than
other events, is observed for different ions at three sta-
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Q. QIAN ET AL. 795
Figure 4. The annual amount wet deposition from 1994 to
2011 in the study area.
Figure 5. The ion concentrations of one typical acid rain
and normal rain.
Figure 6. The number of weekly precipitation events at
tions. It indicates the dilution effect of precipitations on
the dissolved ions extracted from the airborne pollutants.
Figure 7 shows such observations for sulfate and cal-
cium ions at the TX04 station.
The precipitation pH value fluctuates at the stations.
the maximum, average, and minimum values of the field
measured pH were 7.95, 5.50, and 4.31 for the TX04
station, 7.82, 5.18, and 3.97 for the TX16 station and
8.12, 5.44, and 4.05 for the TX22 station, respectively.
To derive a reasonable relationship between pH and the
quantity of precipitation is difficult. The deviation ap-
pears to be caused by the concentration of cations and
anions in the precipitation . Sulfate, nitrate, and chlo-
ride ions presented in the precipitation yield H+ resulting
in low-pH precipitations. On the other hand, calcium,
magnesium, potassium and sodium ions yield OH− re-
sulting in relatively high-pH precipitations. As shown
above in Figure 5, concentrations of sulfate and nitrate
are higher in acid rains (pH ≤ 5.6). On the contrary,
higher concentrations of calcium and sodium are ob-
served to be associated with normal rains (pH > 5.6). It is
worth reporting that the effect of the precipitation quan-
tity on pH is different from acid rain (sulfate-rich) and
normal rain (calcium-rich). As indicated in upper panel
of Figure 8, the measured pH increases with the precipi-
>8070- 8060-7050-6040-5030-4020-3 010-20<10
Pre cipitatio n (mm)
Co n cent ration (mg/l)
95% CI for the Mean
ate ion o n P
>8070-8060-7050-6 040-5030-4020- 3 010-20<10
Precipitatio n (mm)
Concentrat ion (mg/l)
95% CI for the Mean
Calc ium i on on P
Figure 7. The weekly weighted ion concentration of sulfate
and calcium at 95% confidence interval mean versus 10 mm
interval we ekly preci pitations events.
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Q. QIAN ET AL.
Figure 8. pH at 95% confidence interval mean in sulfate-
rich acidic precipitation (upper panel) and in calcium-rich
normal precipitation (bottom panel) at TX04.
tation quantity for acid rains. However, the trend is re-
versed for normal precipitation events as shown in bot-
tom panel of Figure 8. The results confirm the previ-
ously-discussed dilution effect of precipitation on the
concentrations of anions (2
O) and cations
(Ca2+ and Na+) making the water less or more acidic,
3.2. Characteristics of River Water
To characterize the river water, the real time flow rate
data from International Boundary & Water Commission
(IBWC) station 08371500 above the Rio Conchos near
Presidio, station 08374200 below the Rio Conchos near
Presidio and station 08375000 in Johnson Reach in the
Big Bend National Park were collected. The annual hy-
drograph for 08371500 is calculated with the flow data
from 1994 to 2011, and annual hydrographs for 0837
4200 and 08375000 are computed with the flow data
from 1994 to 2007 and 2009 to 2011 since 2008 was a
flood year with large floods (>500 m3/s) from 09/
09/2008 to 10/08/2009. The annual hydrographs for
08374200 and 08375000 are very similar. Figure 9
shows the flow above the Rio Conchos is lower than that
Figure 9. The annual hydrograph in western reach and
below the Rio Conchos and the higher flows occur in
summer. To assess the river water quality, the TCEQ
data of pH, Total Dissolved Solid (TDS), and Dissolved
Oxygen (DO) between 03/05/2010 and 03/05/2013 for
the western reach between El Paso and Presidio above
the Rio Conchos are combined with the flow rate data
from IBWC at the station 08371500. The water quality
and flow rate data at USGS 08374550 near Castolon, and
08375300 in the Big Bend National Park were also col-
lected to study the eastern reach between Presidio below
the Rio Conchos to the Amistad reservoir. The three
years water quality data show the average pH is 7.91 for
western reach and 8.24 for eastern reach. The seasonal
and daily variation in pH is observed. Higher pH occurs
during daylight hours and the summer growing season
when photosynthesis is at its peak . The averaged
DO is 7.73 mg/L for western reach and 7.26 mg/L for
eastern reach. The average TDS is 1746 mg/L for west-
ern reach and 1248 mg/L for eastern reach, and they ex-
ceed the water quality standard permissible value in
Texas. These high TDS values may be caused by irriga-
tion or waste water discharge [21,22].
3.3. Effect of Precipitation on Flow Rate and
To characterize the relationship between the precipitation
and flow rate, the precipitation stations located in the
watershed upstream of USGS 08375300 on the USA side
were considered for hydrological analysis. The daily
point precipitation data were retrieved from the National
Oceanic and Atmospheric Administration (NOAA). The
daily areal precipitation was calculated using the iso-
hyetal method since the topography of the watershed
varies dramatically . The real time flow rate data
from USGS station 08375300 were also collected. As
expected, the daily areal precipitation fluctuates through-
out the period and the flow rate increases after an up-
stream precipitation event. A typical set of observations
between 08/15/2010 to 08/30/2010 is shown in Figure
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Q. QIAN ET AL. 797
10. The precipitation depth = 0.24 inch on 08/16/2010
caused the peak flow rate on 08/17/2010, and the pre-
cipitation depth = 0.16 inch on 08/21/2010 and 0.02 inch
on 08/22/2010 lead to the peak flow rate on 08/23/2010.
The pH = 5.16 for the 0.24 inch event, and pH = 6.19 for
the 0.16 inch and 0.02 inch events can be estimated from
NADP station TX04 weekly precipitation data since the
NOAA precipitation stations are close to TX04 station.
To assess the acid precipitation impacts in the study
area, the three years water quality data analysis indicates
that the pH, DO and TDS decrease with the increasing
flow rate followed by the precipitation event. Moreover,
the DO value is low along with the low pH value, but no
clear correlation between the TDS and pH is observed.
Therefore, the acid deposition decreases the water pH
and may also decrease the DO. Figure 11 shows the wa-
ter pH, DO and TDS values vary with flow rates after
two precipitation events as demonstrated in Figure 10.
The pH and DO values decrease sharply with the in-
crease of the flow rate on 08/17/2010 indicating the acid
precipitation (pH = 5.16) impact on the water quality.
The effect, however, is not as obvious at the precipitation
event (pH = 6.19) on 8/21/2010 and 8/22/2010.
The DO is an important parameter in defining the
health of aquatic ecosystems . DO concentrations
below 5.0 mg/L can produce adverse impacts on aquatic
life . Factors affecting DO are climate or season, the
type and number of organisms and nutrients, organic
wastes, and dissolved or suspended solids . The
drought climate in the watershed can decrease the DO
because water levels decrease and the flow rate of the
river slows down. Low DO after the acid precipitation
event primarily results from the runoff because the runoff
washes out nutrients, dissolved or suspended solids .
Nutrients including nitrate and phosphate are found in
fertilizer runoff. Runoff from roads and other surfaces
can bring salts and sediments into stream water, increas-
ing the dissolved and suspended solids in the water.
Moreover, bottom sediments are stirred by the runoff to
increase the suspended solids in the river .
Figure 10. The relationship between the precipitation and
the flow rate from 08/15/2010 to 08/30/2010.
Figure 11. The pH, DO and TDS values versus the flow rate
from 08/15/2010 to 08/30/2010 at the USGS 08375300.
Following precipitation events, the lowest pH between
03/05/2010 and 03/05/2013 in study area is 6.02, still
close to 6.5, but the lowest DO value is 0.9 mg/L, much
lower than 5 mg/L. Such a low DO can cause serious
water quality problems and can kill the fish in a few
hours. In addition, the slightly higher annual pH and
lower annual DO values are observed in limestone-
dominated eastern reach compared with western reach.
These differences between eastern reach and western
reach may be due to the acid deposition causing the ero-
sion of carbonates, in turn increasing the Ca2+ in the wa-
ter of eastern reach and thereby increasing the pH value.
Copyright © 2013 SciRes. JWARP
Q. QIAN ET AL.
At the same time, the erosion of carbonates adds sus-
pended solids into the runoff, thus in turn decreasing the
DO value. Therefore, the impact of the acid deposition
on the water quality in the eastern reach (limestone-
dominated) is different from the western reach.
The impact of the acid deposition on the water quality
in the limestone basin of eastern reach is a complex
physical and chemical process. To understand better the
limestone erosion from the runoff, an estimation of the
stone loss due to the acid deposition is necessary. After
the runoff reaches the river, the calcium carbonate disso-
lution-precipitation process affects the water quality pa-
rameters, such as the hardness, pH, DO and temperature.
The dissolution reactions of calcium carbonate (CaCO3)
occurring in parallel are as follows :
CaCO (s)+H COCa+2HCO
The dissolution rate has been described as 
The dissolution rate is a function of the pH, PCO2 and
the reaction rate constants. At very low pH, the rate of
dissolution is fast, but within the pH range of natural
waters the dissolution rate is controlled by the chemical
reaction at the water-mineral interface with a very small
dissolution rate (<10−8 mol cm−2sec−1). However, with
wet and dry deposition of SO2 and HNO3, an important
consequence is the rapid weathering of exposed lime-
stone. Lipfert  has proposed a theoretical damage
function equation to describe the weathering of limestone
between pH 3 and 5:
dS 2 dN3
Stone lossμm/m/y = 18.8+0.016 H
+0.18 VSO +VHNOR
where (H+) is in µmol/L, VdS and VdN are impact veloci-
ties of SO2 and HNO3 on stone surface in cm/s, atmos-
pheric concentrations of SO2 and HNO3 are in µg/m3,
and R is meters of rain per year.
The erosion increases with the acidity, the velocities
and concentrations of SO2 and HNO3. Recalling the an-
nual wet deposition data (1994 to 2011) obtained from
NADP, the concentration of H+, atmospheric concentra-
tion of SO2 and HNO3, and the rain per year can be cal-
culated. The VdS and VdN are estimated to be around 0.2
cm/s ~ 2 cm/s with the American Society of Civil En-
gineering kinematic wave equation in . The average
annual stone loss is about 0.02 mm. If only 10% of the
limestone is bared soil, the total stone loss in the sub-
basin is estimated around 0.02 mm x 34630 sq miles ×
40% of limestone × 10% bared soil = 71746 m3 (72,000
m3). If only 1% of the stone loss can reach the river by
the runoff, a total sediment load of 717 m3 will be added
to the river. The erosion leads to increased sediment
storage, decreased channel capacity and increased flood-
ing frequency and the water quality (e.g. TDS) exceeds
the maximum permissible value set by Texas State Water
Quality Standards. The ongoing effort is to evaluate the
sediment transport in the channel.
3.4. Daily pH Fluctuations Caused by Calcium
The presence of dissolved calcium carbonate causes in-
creasing hardness. The Langelier Saturation Index has
been developed to predict the tendency of calcium car-
bonate either to precipitate or to dissolve under varying
conditions . With a pH of 6.5 to 9.5, the equation
expresses as :
where pHs = the pH at which water with a given calcium
content and alkalinity is in equilibrium with calcium car-
bonate, K2 = the second dissociation constant for carbo-
nic acid, Ks = the solubility product constant for calcium
carbonate. These terms are functions of temperature and
total mineral content.
Daily pH fluctuations were observed in river water
under steady flow rates. The fluctuations were found to
synchronize with temperature fluctuations. A typical set
of such observations is displayed in Figure 12. The phe-
nomenon is believed to be due to the effect of the oscilla-
tion of the limestone dissolution-precipitation process.
With limestone being the major component (55%) in the
subbasin of the river segment, calcium carbonate (CaCO3)
is expected to be abundant in the water with the dis-
solved amount near the solubility amount or the 2
concentration (proportional to pH) near the saturation
concentration (or saturation pH), which decreases with
temperature . The observed daily pH fluctuations
appear to reflect the limestone dissolution process near
the equilibrium concentration at the daily temperature
Figure 12. The relationship between water pH and tem-
Copyright © 2013 SciRes. JWARP
Q. QIAN ET AL. 799
fluctuation. The data shown in Figure 12 indicate that as
the temperature starts to increase from its daily low point,
the pH also increases due to the increase in dissolution
kinetics . However, at a point, while the temperature
continues to increase, the limestone dissolution hits its
equilibrium point and has to start to precipitate causing
the pH to decrease. This continues until the daily tem-
perature reaches its maximum. Then, once the tempera-
ture starts to decrease, the pH continues to decrease until
the temperature reaches its daily minimum. The process
then repeats itself with the next temperature fluctuating
It should be noted that the pH value at equilibrium is a
function of the water temperature, the solubility of
CaCO3, and the concentration of total inorganic carbon
and alkalinity in the water . An attempt was made to
simulate the equilibrium pH at different existing inor-
ganic concentrations employing GWB (“Geochemist’s
Work Bench” developed by University of Illinois at Ur-
bana-Champaign) software and the results are shown in
Figure 13. It demonstrates the equilibrium pH decreases
with decreasing temperature, but increases as the CaCO3
concentration decreases. The equilibrium pH values ex-
tracted from the daily pH fluctuation data described in
Figure 12 are also presented in Figure 13. The results
indicate that the observed equilibrium pH is in agreement
with the simulated equilibrium pH with initial CaCO3
concentrations between 80 and 400 mg/L. The effect of
limestone equilibrium on the observed daily pH fluctua-
tions appear to be supported by the simulation.
A study assessing acid deposition effect on the water
quality in the upper Rio Grande River section in Texas
between El Paso and the Amistad Reservoir has been
carried out. The study has included the collection and
analysis of field measurement data and the performance
of hydrological and water chemical analyses to assess the
impact of acid deposition on the water quality. The data
analyzed have included the amount of precipitation and
Figure 13. The water pH versus the temperature at differ-
ent CaCO3 concentration.
the corresponding water properties in the precipitation,
including pH and concentrations of , 3
Na+, and other species, as well as the corresponding wa-
ter quality properties in the river water.
The results from the analysis of the precipitation data
indicate that the concentrations of sulfates, nitrates, chlo-
rides, ammonium, calcium, magnesium, potassium, and
sodium decrease with an increase in the amount of the
precipitation. Lower pH values have been observed to be
associated with higher concentrations of sulfates and
nitrates ions in the precipitation. On the contrary, higher
pH values have been associated with higher concentra-
tions of calcium and sodium ions.
The results from the analysis of river flow data indi-
cate the higher flows occur in summer and the flow
above the Rio Conchos is lower than that below the Rio
Conchos. The water quality analysis demonstrates that
the pH value, the Dissolved Oxygen (DO) and the Total
Dissolved Solid (TDS) in the river generally decreases
when the flow rate increases immediately following pre-
cipitation events. The drop in pH following a precipita-
tion event is due to the low pH in the precipitation. The
DO and TDS decreases after the precipitation are caused
by the increased flow rate. The lower DO value along
with lower pH indicates that the acid deposition not only
decreases the water pH, it may also decrease the DO
value. The slightly higher pH and lower DO values in the
reach downstream of the Rio Conchos to the Amistad
Dam are due to the rich limestone eroded by the acid
deposition. The annual stone loss by the acid deposition
in the basin has been estimated about 72,000 m3. The
higher pH and higher hardness are expected due to cal-
cium carbonate presence. The data also indicate the daily
river pH values fluctuate with the daily river temperature
suggesting the effect of CaCO3 solubility. To estimate
the effect of the oscillation of limestone dissolution-pre-
cipitation process, water chemistry analysis using Geo-
chemist’s Work Bench (GWB) was performed. The equi-
librium pH decreases with decreasing temperature, but it
decreases with increasing CaCO3 concentration. The ef-
fect of limestone on observed daily pH fluctuations ap-
pears to be supported by the simulation.
The authors are grateful for the financial support of this
study from the United States Department of Agriculture
through the Sul Ross State University under USDA
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