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Weathering and water quality in the Blanco River, a subtropical karst stream.

Abstract. -- The Blanco River is a karst stream that traverses the Texas Hill Country and Balcones Fault Zone regions associated with the eastern Edwards Plateau. It is closely connected with the Trinity Aquifer, the Balcones Fault Zone Edwards Aquifer, and the San Marcos River, all being extremely valuable regional resources. This study characterizes water quality in the Blanco River from the headwaters to mouth, with particular attention given to the effects of spring and tributary inputs on concentrations of dominant ions calcium ([Ca.sup.2+]), magnesium ([Mg.sup.2+]), and bicarbonate (HC[O.sub.3.sup.-]). Historical correlations between discharge and concentrations of specific ions are considered as evidence of active diagenesis, specifically dedolomitization, in rock units of the Blanco River. A greater concentration of dolomitic weathering products in the headwaters region also supports this conclusion.


Karst rock, including silicates, evaporites, and carbonates have a higher degree of solubility in natural waters than other rock types (Gunn 1986). Carbonates such as calcite and dolomite are the most commonly occurring of these, and are the minerals of primary interest in this study. The dominant erosive process involved in karst geomorphology is chemical weathering, and is largely driven by carbonic acid, which is formed in the water or soil solution from C[O.sub.2] addition. This slightly acidic solution dissolves the limestone or dolomite crystal lattice. The ion chemistry of rivers and streams in karst areas is normally dominated by constituents of the local limestone in high concentrations. Over time, infiltration of water into bedrock allows for the growth of cracks and channels that can transport larger amounts of flow over longer distances underground. When geological faulting and folding also occur, the dissolution of karst rock often creates ideal conditions for the formation of aquifers. Aquifer systems are a source of water for humans and also give rise to springs and seeps that often support a diverse biota, including populations of endemic organisms.

The Blanco River is a little-studied karst stream in the eastern part of the Edwards Plateau region of central Texas. Approximately 140 km in length, it descends 250 meters in elevation and passes through the three towns of Blanco, Wimberley, and San Marcos, Texas (Guadalupe-Blanco River Authority 1961). The river's 1,311 [km.sup.2] drainage basin crosses two different regions of the Edwards Plateau (or the Edwards-Trinity Aquifer), the Texas Hill Country and the Balcones Fault Zone (Barker & Ardis 1996). Seepage from streams draining to the east and south through the Hill Country provides recharge to the Balcones Fault Zone Edwards Aquifer; median annual recharge from the Blanco River between 1934 and 2004 was estimated to be 4.40 x [10.sup.7] [m.sup.3] x [yr.sup.-1] (35,700 acre-feet x [yr.sup.-1]), or about 6% of the total recharge to this aquifer (Edwards Aquifer Authority 2005).

The Balcones Fault Zone Edwards Aquifer supplies water to large human populations; in 1975 it was designated the first sole-source aquifer in Texas for the city of San Antonio (Bowles & Arsuffi 1993). Subterranean flow in the aquifer moves mostly east-northeast and discharges naturally at the Comal, Hueco, and San Marcos springs. The San Marcos Springs support many endemic or range-restricted organisms. The Blanco River supplies water to the San Marcos River through both ground and surface water pathways. Potentiometric surface analysis and water loss studies indicate that significant spring recharge comes directly from the lower Blanco River (Ogden et al. 1986). The Blanco River flows into the San Marcos River 7.2 km downstream from the springs. There the larger size of the Blanco River watershed and the associated terrestrial inputs has a significant effect in greatly increasing variability in physical and chemical characteristics downstream of their confluence (Groeger et al. 1997).

Like many of the rivers and streams of central Texas, the Blanco River is prone to fast moving, large volume flows created by the short duration, heavy precipitation events typical to the region. Rivers in central Texas are among the flashiest and most variable perennial rivers in the world (Slade 1986; Poff & Ward 1989; Groeger & Bass 2005). Conversely, flows cease for extended periods in some sections of the river, and what surface flow remains in other areas may be entirely dependent on groundwater emerging from numerous seeps and springs along the river's course as well as tributary surface flows. The Blanco River channel is intersected by several normal faults, and resulting fractures, bed displacements, and other structural changes have led to a short-circuiting of surface flows. The greatest loss in stream flow in the Blanco River occurs between 84 and 73 river km from the mouth, where water infiltrates the bedrock in an area of faults and highly fractured limestone created by an anticlinal flexure. According to a previous investigation, water moves in "open fractures or joints through the channel-entrenched carbonate" to appear as baseflow gain downstream (Buckner & Thompson 1964). Other sections of discontinuous surface flow are interspersed along the Blanco River drainage.

The two major tributaries of the Blanco River are the Little Blanco River and Cypress Creek. The Little Blanco River intersects the Blanco River 77 km from the mouth, and it too depends on underground flow paths. Several persistent pools that ostensibly originate from spring inputs were observed downstream of the monitoring site when upstream flows were nonexistent. Cypress Creek is a 43 km stream (only the lower 22 km are perennial) which flows through the city of Wimberley to meet the Blanco River approximately 48 km upstream from the mouth (Bonner et al. 2002). Cypress Creek is fed by a large limestone spring known as Jacob's Well (DeCook 1963). The Jacob's Well springs stopped flowing temporarily for the first time in recorded history in the summer of 2000, a phenomenon attributed to drought and increased development in the Wimberley area.

The combination of above and below-ground flow paths in the mainstem and tributary Blanco River drainages should produce unusual relationships between discharge and the dissolved solute load. In river systems, concentrations of dissolved ions have a strong tendency to decrease in response to increasing discharge (Walling & Webb 1986; Meybeck 1996). During higher flows, runoff is moved quickly to and through the channel and has little time to pick up solutes. Some rare cases exhibit a concentration rather than a dilution effect in response to increasing discharge. There was a positive correlation between discharge and specific conductance in four of the streams contributing recharge to the Balcones Fault Zone Edwards Aquifer (Groeger & Gustafson 1994). Two different causes for this phenomenon in other systems have been suggested. Rains following an extended dry period may act as a flushing mechanism for solutes accumulated in the channel during dry periods in summer. Alternatively, base flows in contact with a lower rock unit could have a more dilute chemical signature than when the water table rises and comes into contact with more soluble formations. There have been no studies to investigate this phenomenon in subtropical karst systems, where climate, geology, and hydrology interact in a regionally unique way.

This study sought to characterize water quality in a subtropical karst stream from headwaters to mouth to determine current baseline conditions, and in conjunction with other scientists establish the response and distribution of the riverine biological community to the physical and chemical components of the ecosystem. This is especially important in light of rapidly growing local human populations and the regional stress they place upon surface and groundwater resources.


Eight sites on the Blanco River were selected for this study with consideration given to position, accessibility, proximity to urban areas, and representation of different microhabitats such as riffles, runs, or pools (Fig. 1). Site names correspond to distance upstream from the river mouth in river km (Guadalupe-Blanco River Authority 1961). Elevations of study sites and significant landmarks taken from the survey were compared with readings taken with a handheld GPS unit and found to be similar. Sites were sampled on 18 dates (monthly) from November 2003 through July 2005.

Site 127 was adjacent to private property, upstream of Blanco, Texas and downstream of several perennial seeps that meet the river on the property. Site 117 was located at Wayne Smith Lake, one in a series of small overflow impoundments upstream of the Highway 281 Bridge in Blanco. Sites 101 and 71 were adjacent to private property between Blanco and Wimberley, Texas. Site 42 was located in Wimberley approximately 6.4 km downstream from the confluence with Cypress Creek. Sites 14 and 6 were located at road crossings in the city of San Marcos. Additional sites on the Little Blanco River and Cypress Creek were also selected for this study. The Little Blanco River study site was located approximately 1 km upstream from the confluence with the Blanco River. The Cypress Creek site was located approximately 0.87 km upstream from the confluence with the Blanco River.


Measurements and discrete samples were made from the bank in areas with stream flow. Temperature, pH, dissolved oxygen, and specific conductance were measured using a Hydrolab[TM] Minisonde calibrated one day prior to each sampling event. Alkalinity was measured by potentiometric titration to pH 4.8 using 0.02 N [H.sub.2]S[O.sub.4] (Wetzel & Likens 2000). Turbidity was measured using a Fisher Scientific Turbidometer. Calcium ([Ca.sup.2+]) and magnesium ([Mg.sup.2+]) were measured by atomic absorption (American Public Health Association 1998). Soluble reactive phosphorus (SRP) was determined using the ascorbic acid method (Murphy & Riley 1962). Nitrate-nitrogen (N[O.sub.3]-N) was measured by second-derivative UV spectroscopy (Crumpton et al. 1992).

Historical discharge and chemistry data were obtained from United States Geological Survey (2005). Temperature was recorded every six minutes at Sites 127, 101, and 6 from 20 May 2004 to 20 September 2004 using StowAway[R] XTI Temperature Loggers (Onset Computer Corporation). The devices were placed in the best developed channel to ensure constant immersion and shaded locations were selected to prevent heating from direct contact with sunlight.

The section of river between Sites 101 and 71 had no visible surface flow from 9 January 2004 to 14 March 2004. Surface flow was observed to have ceased in the same area in the second week of June 2005 and remained dry for the remainder of the study. A large spring located downstream from Site 71, Valley View Spring, appeared to be the initial source for surface flows downstream from this area; however, additional springs with openings smaller than 1 cm were observed nearby emerging from the bedrock of the river channel. The orifice of Valley View Spring was much larger, approximately one meter in width. When accessible, water from the spring was sampled and analyzed in 2004 and 2005. Temperature loggers described above recorded temperatures at Valley View Spring and Site 101 every 6 minutes from 23 June 2005 to 11 September 2005.


In 2004, a relatively wet year, Blanco River flows were greater than the 3rd quartile of historical levels from March through December. In 2005, flows approached median historical levels after March and continued to decrease for the remainder of the period of study. Several high discharge events were sampled, including Site 6 on 30 June 2004, when mean daily discharge was 105 [m.sup.3]/s (3,708 cfs). Surface flows in the Little Blanco River were typically nonexistent in the summer at the site described above, and therefore the data from the Little Blanco River reflects a winter sampling bias.


Mean water temperatures from the thermistors placed at Sites 127, 101, and 6 showed a clear warming trend from upstream to downstream during the summer of 2004, corresponding to an increase of about 1.1[degrees]C over 121 km, or a loss in elevation of about 180 m (Table 1). Median daily temperature ranges and variability both decreased moving downstream. Dissolved oxygen in the Blanco River tended to be near saturation with the atmosphere, with 82% of dissolved oxygen readings between 80 and 120% saturation (Fig. 2, Table 2). Diel sampling in the river also suggested that night time oxygen did not drop so low as to be harmful to stream organisms (Cave 2006).

Stream turbidity was consistently low, except at Site 117 (Wayne Smith Lake) where it was usually three to four times higher than that of other Blanco River sites (Fig. 3a). The high mean turbidity at Site 6 reflects extremely high values recorded during storm flows on 6 June 2004 and 30 June 2004. Specific conductance did not have a discernable trend from headwaters to mouth (Fig. 3b). Of the three dominant ions measured in this study, specific conductance was most strongly correlated with alkalinity ([r.sup.2] = 0.43). Alkalinity was highest at Site 127 and decreased moving downstream (Fig. 3c). Median Ca:Mg ratio for the Blanco River was 1.44, but upstream of Site 71 the ratio was usually closer to 1:1 (Fig. 4a). The combined charge of the two cations was typically equivalent to measured alkalinity. The highest median concentration of SRP was observed at Site 71 (Fig. 4b). The greatest concentrations of N[O.sub.3]-N were found at Sites 117, 101, and 71 (Fig. 4c).


Compared to the Blanco River, water emerging from Valley View Spring was low in pH and dissolved oxygen, but high in specific conductance, alkalinity, [Ca.sup.2+], SRP, and N[O.sub.3]-N (Table 2). The thermistor record reflects a time when surface flows were non-existent between the two locations and the spring system appeared to be the origin of flows downstream. Median water temperature of the spring water was more than 6[degrees]C cooler than Site 101 and varied very little over this period (Table 1).


Data collected by the USGS indicated that specific conductance, [Ca.sup.2+], and HC[O.sub.3.sup.-] were lowest in summer months, but [Mg.sup.2+] showed little variation from month to month (Fig. 5a). Concentrations of [Ca.sup.2+] and HC[O.sub.3.sup.-] significantly increased with increasing discharge ([r.sup.2] = 0.29 and 0.31, respectively, p<0.05), while [Mg.sup.2+] decreased ([r.sup.2] = 0.54, p<0.05) (Fig. 5b). Specific conductance was not significantly related to discharge in this historical data.



Diel dissolved oxygen concentrations indicated that there was not a problematic imbalance between photosynthesis and respiration within the river. Greater and more variable concentrations of SRP and N[O.sub.3]-N found at Sites 101 and 71 may have anthropological origins; however, patterns in historical land use within the drainage basin were not investigated for this study. Alternatively, spring inputs like those from Valley View Spring enriched river water in N[O.sub.3]-N, though much less so than the waters from the Balcones Fault Zone Edwards Aquifer emerging at the Comal or San Marcos Springs (Groeger & Gustafson 1994). Dissolved oxygen and nutrient concentrations suggest that anthropogenic eutrophication of the river system has been for the most part slight. With the very low surface flows common during the typical drier summer months, point sources of nutrients would have severe consequences for the health of this river ecosystem.

The weathering of dolomite or magnesium-rich limestone seems to be especially influential on solute concentrations in the Blanco River. Springs draining dolomites or dolomite-related rocks have a Ca:Mg ratio near unity, while that of springs draining limestone rocks are 3 to 7 times that (Shuster & White 1971). Generally, limestone containing dolomite is much less soluble than those having a higher concentration of calcium (Gunn 1986). Increased solubility of dolomite at higher temperatures, however, has allowed the development of striking karst landforms in tropical regions. Suspended and settled precipitates observed in the Blanco River are probably entirely calcitic, as magnesium fails to precipitate even under extremely high saturation conditions (Land 1998). The low median Ca:Mg ratio indicates that the most recent aggressive dolomite solution in the drainage was probably taking place in the headwaters. The elevated concentrations of [Mg.sup.2+] upstream appeared to be diluted downstream by waters draining rock units richer in calcite.

A portion of the river's water is supplied by two major tributaries, which are both, for the most part, similar in origin and chemical character to the Blanco River. The intermittent Little Blanco River tributary seemed to have little effect on Blanco River chemistry during the study, but joins the Blanco River in the region in which both were dry under low flow conditions. Therefore the upstream Little Blanco River surface flow, which is usually present, is probably entering the same or a parallel underground flow pathway through which the upstream Blanco River water has moved. Water from Cypress Creek, which is often quantitatively important during low flow periods (Buckner & Thompson 1964), had an observable effect on the Blanco River. Elevated pH, specific conductance, and [Ca.sup.2+] recorded at the site downstream of the confluence (Site 42) probably resulted from solute-rich inflows from spring-fed Cypress Creek. The increase in solute concentrations could also have been influenced by spring outflows in the Blanco River channel upstream of the confluence. A significant stream flow gain was traced to springs in an area between 18.3 and 19.3 km upstream from the mouth of Cypress Creek (Buckner & Thompson 1964).

Groundwater from springs like Valley View Spring may be a significant driver for temperature and dissolved ion dynamics in the Blanco River. Valley View Spring water was cooler than Blanco River surface water in the summer and warmer in the winter, indicating that this water had a much slower underground transit time than above ground. Groundwater was found to be an important source and sink for thermal energy for a Pennsylvania karst stream, depending on season, and stream temperatures were strongly related to surface and groundwater interactions (O'Driscoll & DeWalle 2006). Chemically, the spring water was enriched in [Ca.sup.2+] and alkalinity relative to Blanco River surface water and was lower in pH. The lower pH in the spring water suggests that C[O.sub.2] introduction is occurring more rapidly than the process of calcite or dolomite dissolution (Thrailkill 1972).

Texas surface waters show a strong trend of decreasing specific conductance from west to east corresponding to increased rainfall (Groeger & Ground 1994). However, in the Edwards Plateau region, those systems farthest to the east exhibited higher specific conductance than those to the west (Groeger & Gustafson 1994). The drainages in this region clearly possess hydrological and geomorphological features that exhibit a unique chemical response to changing hydrological conditions. Storm flow data suggest a threshold for the positive relationships between discharge and concentrations of [Ca.sup.2+] and HC[O.sub.3.sup.-]. Samples from Site 6 during high discharge events on 10 June 2004 and 30 June 2004 do not reflect a proportional increase in dissolved solute concentration, but rather they were more dilute with respect to median values from 2003 to 2005. However, samples associated with storm flows were obtained on the falling limb of the hydrograph, and it was not determined whether this was the late stage of a "flushing" effect.

Summer decreases in concentrations of [Ca.sup.2+] and HC[O.sub.3.sup.-] probably result from a combination of lower discharge and higher daily temperatures. Solubility of calcium carbonate decreases with increasing temperature (Drever 1997). Historically, [Ca.sup.2+] and HC[O.sub.3.sup.-] in the Blanco River decreased with increasing temperature (Fig. 5A, [r.sup.2] = 0.43 and 0.42, respectively, p<0.05). There was no relationship between [Mg.sup.2+] concentrations and temperature in the historical data.

The process of active diagenesis in Blanco River rock units could explain both the relationships between discharge and dominant ion concentrations as well as the greater abundance of dolomitic weathering products in the headwaters. In the late Miocene period, faulting along the Balcones fault zone raised the Edwards Plateau in the north and west relative to sea level and enabled the formation of a circulating freshwater aquifer (Ellis 1986). Since that time, rocks in the aquifer underwent several major near-surface diagenetic changes, including extensive dedolomitization. In this process, freshwater flushing replaces gypsum and magnesium in dolomitic rocks with calcite. The resulting dedolomite can be more soluble than the original dolomite (Evamy 1967). Isotopic ratios suggest dedolomitization continues in the present-day (Ellis 1986). High concentrations of sulfate, possibly resulting from dissolved gypsum, have been recorded in the upper Blanco River (Guadalupe-Blanco River Authority 2003). Sulfate concentrations, like [Mg.sup.2+], exhibited a negative relationship with flow. Increased discharge could allow water in subterranean flow paths to reach more soluble, magnesium-poor calcites, creating the concentration effect observed for [Ca.sup.2+] and HC[O.sub.3.sup.-] in the historical record (Fig. 5a). During storm flows, the amount of water flowing through the system overwhelms the concentration effect and dissolved solutes become diluted.

The streams recharging the Balcones Fault Zone Edwards aquifer are similar in chemistry (Groeger & Gustafson 1994). Like the Blanco River, spatial and temporal patterns in the ion content of each stream are presumed to be driven by the physical and chemical weathering processes occurring in their underlying geological forms. Past and present data suggest that these streams would not exhibit higher salinities in a drier climate, thanks in large part to unique solution mechanics affecting the local karst and the tight connectivity between surface and groundwater flow paths. Endemic aquatic organisms depend on water from springs and seeps to maintain base flows as well as the consistent thermal and chemical conditions to which they have adapted. The perpetuity of these valuable ecosystems and their associated biotic assemblages will depend on human diligence in preserving the supply of environmental groundwater flows.


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AWG at

Michael S. Cave and Alan W. Groeger

Department of Biology/Aquatic Station, Texas State University-San Marcos

601 University Drive, San Marcos, Texas 78666
Table 1. Median temperature and median daily temperature range
([degrees]C) at Sites 127, 101, and 6 from 20 May 2004 to 2 September
2004, and at Site 101 and Valley View Spring from 23 June 2005 to 11
September 2005. SD = standard deviation.

 20 May 04-2 Sept 04
 Site 127 Site 101 Site 6

Water Temperature 27.13 27.24 28.29
 SD = 2.05 SD = 1.77 SD = 1.51
Daily Range 3.30 2.99 1.65
 SD = 1.85 SD = 0.70 SD = 0.56

 23 Jun 05-11 Sept 05
 Site 101 Valley View Spring

Water Temperature 29.24 22.98
 SD = 1.54 SD = 0.89
Daily Range 3.32 0.17
 SD = 1.13 SD = 0.11

Table 2. Median values for selected variables measured in the Blanco
River, Little Blanco River, Cypress Creek, and Valley View Spring from
2003 to 2005. SD = standard deviation; n = number of measurements.

 Blanco River Little Blanco River

Temperature ([degrees]C) 22.98 19.38
 n = 129, SD = 6.36 n = 5, SD = 6.38
pH 7.92 7.59
 n = 129, SD = 0.17 n = 5, SD = 0.15
Dissolved [O.sub.2] (mg/L) 8.66 8.00
 n = 124, SD = 1.51 n = 5, SD = 1.60
Sp. Cond. ([micro]S/cm) 439 455
 n = 129, SD = 49 n = 5, SD = 52
Turbidity (NTU) 3.1 2.3
 n = 128, SD = 6.7 n = 6, SD = 0.5
Alkalinity (meq/L) 3.93 4.30
 n = 128, SD = 0.54 n = 6, SD = 0.42
Calcium (meq/L) 2.32 2.56
 n = 111, SD = 0.29 n = 4, SD = 0.17
Magnesium (meq/L) 1.60 1.60
 n = 111, SD = 0.32 n = 4, SD = 0.26
Ca : Mg 1.44 1.56
 n = 111, SD = 0.39 n = 4, SD = 0.45
SRP ([micro]g/L) 4.1 2.0
 n = 104, SD = 6.37 n = 4, SD = 7.2
N[O.sub.3]-N ([micro]g/L) 316 196
 n = 100, SD = 234 n = 4, SD = 95

 Cypress Creek Valley View

Temperature ([degrees]C) 19.98 23.08
 n = 17, SD = 4.22 n = 7, SD = 4.75
pH 7.68 7.26
 n = 17, SD = 0.14 n = 7, SD = 0.16
Dissolved [O.sub.2] (mg/L) 8.83 6.25
 n = 16, SD = 1.12 n = 7, SD = 1.91
Sp. Cond. ([micro]S/cm) 522 475
 n = 17, SD = 38 n = 7, SD = 47
Turbidity (NTU) 2.1 2.1
 n = 17, SD = 0.7 n = 6, SD = 0.4
Alkalinity (meq/L) 4.92 4.26
 n = 17, SD = 0.53 n = 6, SD = 0.29
Calcium (meq/L) 2.95 2.72
 n = 15, SD = 0.36 n = 5, SD = 0.44
Magnesium (meq/L) 1.60 1.55
 n = 15, SD = 0.35 n = 5, SD = 0.57
Ca : Mg 1.74 1.59
 n = 15, SD = 0.57 n = 5, SD = 0.49
SRP ([micro]g/L) 3.2 5.1
 n = 14, SD = 3.1 n = 4, SD = 0.81
N[O.sub.3]-N ([micro]g/L) 209 452
 n = 13, SD = 115 n = 4, SD = 79
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Author:Cave, Michael S.; Groeger, Alan W.
Publication:The Texas Journal of Science
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2007
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