GEO 371C GIS and GPS Applications and Earth Sciences

Bull Creek Watershed

Impacts of urbanization on water chemistry in flowing streams of the Bull Creek Watershed

Austin, Texas

INTRODUCTION

The Bull Creek watershed is located in north West Austin, its 25 square miles drain into the

Colorado River at Lake Austin. It has an 11 mile creek with scenic cliffs and waterfalls. The

watershed includes limestone seeps, springs, and waterways, which makes it the perfect home

to a number of indigenous species of flora and fauna. All of these characteristics make the Bull

Creek Watershed a very attractive area for land developers. Land development has increased

in the last several years, according to the City of Austin in 2000 the population of Bull Creek was

43,709 and they project that by 2030 the population will be 69,716. Urban development

increases surface impervious area, which decreases infiltration rates. The recharge of urban

ground water is heavily affected by extensive sealing of surfaces and leaky water mains. At the

same time that urbanization increases so does the underground pipe network used to carry

potable and wastewater, the risk of leaky pipes increases with the condensation of water pipe

networks. This paper deals with the assessment of potential risks due to damaged main water

pipes, and to find a correlation between the damaged water pipes systems and underground

water chemistry. The main focus of this paper is to analyze how main water pipes densities

within sub-water sheds of the Bull Creek watershed affect the concentrations of Na+Cl and

Ca+HCO3.

Figure 1. Picture of one of the few flowing springs (Picture by Daniel Reyes).

DATA GATHERING

Data for this study was collected from the following sources:

1- City of Austin

ftp://ftp.ci.austin.tx.us/GIS-Data/Regional/coa_gis.html

Creek Lines: Shapefile (vector digital data) Geographic coordinate system name: GCS North American 1983.

Combination of stream centerlines created from aerial orthography,

planimetrics based on LIDAR data “Hydro_L ”(a non-continuous stream) and

construction plans which has been updated and edited to be continuous stream.

Data created by TWDB.

Water Shed: Shapefile (Poligon vector digital data) Geographic coordinate system name: GCS North American 1983

Polygon shape file showing areas of all of the major watersheds in

central Texas.

I was unable to access metadata for this shapefile thus it was not

specified who the author is.

Main Water Pipe: Shapefile (vector digital data)

Map Projection Name: GCS North American 1983

Geographic coordinate system name: GCS North American 1983

of all installed water pipes, year installed, and length of pipe network within the

Bull Creek watershed.

In order to obtain these data I had to contact Kevin J. Smith, Sr. GIS

Analyst from City of Austin, Watershed Engineering Division.

Kevin.smith@austintexas.gov

2- Daniel Reyes (Variations in springwater geochemistry in a rapidly urbanizing watershed)

Water Shed Deliniation: shape files (Poligon vector digital data)

Map Projection Name: GCS North American 1983 Harn

Shapefile was created by using flow direction, flow accumulation, stream

definition, stream segmentation, catchment grid delineation. It separates Bull

Creek into smaller subwatersheds, these subwatersheds are assumed to be

the main contributor to each of the sampled sites.

Daniel Reyes constructed this shapefile with the help a lab from the class

GIS in Water Resources, thought by David R. Maidment. The linear unit for

this file was in meters in contrast with to the rest of the shapfiles from city of

Austin which uses feet.

Water Spring Sites: shape files (Point vector digital data)

Map Projection Name: GCS North American 1983 Harn

This shape file has all of the geographic locations of all of the springs

that were tested for this study.

The linear unit for this file was in meters in contrast with to the rest of

the shapfiles from city of Austin which uses feet.

Water Chemistry: excel file

Data showing chemical compositions of all of the sites tested.

The University of Texas at Austin. Cation (Sr, B, Ca, Mg, K, Na)

concentrations have been measured by quadrupole Inductively coupled

plasma mass spectrometer (ICP-MS) at the Lower Colorado River Authority

(LCRA) facility in west Austin. Anion (F, NO3, Cl, SO4) concentrations have

been measured by Ion Chromatography at the LCRA. The soil samples have

not yet been analyzed in the TIMS.

METHODS

Project data in common predefined coordinate system

Using analysis tools create a shapefile which contains the Bull Creek area by itself

Using analysis tools crate a shapefile with streams contained within Bull Creek watershed

Join Water Chemistry Excel file with the Water Spring Sites

Find sub-watershed areas by using the select by location tools

Find pipe total length within sub-watershed using the select by attributes tools

Find pipe density by dividing total pipe density by the area of its correspondent sub-watershed

In three different maps show pipe density, Ca+HCO3 and Na+Cl conentrations

Create two graphs, Pipe density vs Ca+HCO3 and Pipe density vs Na+Cl concentrations

An increase in Na+Cl concentration should be evident in relation to Ca+HCO3, mainly due to water softening treatment by the city water management

H2O + CaCO3 + CO2 = Ca2+ + 2HCO3

-

Data Processing

Projections

There were some issues with projecting all of the layer files at the beginning of the project. This

was mainly due to unknown metadata coordinate systems from some of the files. This could be

easily fixed by choosing the correct coordinate system. No changes adjustments of the

coordinate system were made, since the problematic files were not needed for to solve the

problem of this project.

Isolating Bull Creek

Isolating Bull Creek shape feature was relatively easy using analysis tools from the ArcToolbox

pull down menu. The best and easiest way to extract Bull Creek is to extract the data by its

attributes, in this case by a query that specifies its name has to be created.

Figure 2. ArcTollbox for selecting features

The Select tool under the Extract tools brings up a new window where an input feature has to be

specified, in this case the watershed polygon file. The output feature class pull down menu allows

specifying where the new shape file will be saved. For selecting the Bull Creek water shed a simple query

had to be created: DCM_NAME=Bull Creek, this query tells the computer to only extract the polygon

with the attribute name Bull Creek. This Polygon will be used in the next step.

Figure 3. Query Builder from Extract-Select Toolbox.

Figure 4 shows Bull Creek watershed before selection. Figure 5 shows Bull Creek after extraction.

Cliping Streams

In this step the Clip tool is used to clip the features within the Bull Creek watershed polygon. With this

technique the polygon created in the previous step is used to specify the area of interest. The impute

data to be extracted must meet some criteria, it must fall completely within the Bull Creek watershed.

Figure 6. Clip tool from the Extrct Toolbox.

The following window (fig. 7) is prompted by selecting the Clip tool. In this window the input features

and the clip features must be specified. A new feature is created by preassing the OK button, the new

feature contains all of the same attribute values as the input features.

Figure 7.

Figures 8 shows the Creeks data file before being clipped and figure 9 shows the same data after

clipping.

Figure 8

Figure 9

Joining Tables

Joining tables is a very simple process by using the joins and relates tool. The following window is

prompted, this window allow to brown for the excel table containing the chemical data for all of the

springs that where analyzed. All of the fields used to create the joint must be identical otherwise the

new attribute table will show null values for all the non-matching cells.

Figure 10

Calculating Densities

As the class progressed it became clearer that ArcMap has many different ways of analyzing

data. It is important to know that some ways of analyzing data are more efficient than others,

you can save time and get better results by choosing the correct technique. The selection menu

is one of those tools which allows to select features in many different ways. In this step two

selection techniques were used. Select by attributes allows to selects each of the

subwatersheds individually. The purpose of selecting each watershed individually is to highlight

those water pipe lines that fall within each area.

Figure 11 Selecting Subwatersheds by Attributes.

Figure 12 Selecting water pipes by location.

Figure 12 shows the highlighted pipe length within a specific subwatershed. The same data is highlighted

in the attribute table for the same layer. The total length of the highlighted water pipe within each

subwatershed was obtained from the attribute table. The attribute table has a selection tool, under the

selection tool there is a statics option which gives you a list of statistical values (see figure 13).

Figure 13

The cell containing the sum of the total pipe length is then copied in an excel table in order to calculate

the pipe density in relation to the area of each sub watershed.

RESULTS

Table one was used to create the results of figures 14 and 15. Figures 16, 17 and 18 were color labeled

to with the same colors to represent density chemical concentrations. The purpose of using same colors

was to show differences within each of the sub-watersheds.

ID Na+Cl(mg/l) Ca+HCO39(mg/l) areaft pipelenght (ft) pipedensity

Tubb 47.4 618.68 158957.6579 2691.965005 0.016935107

Fern Gully 30.8 472.8 172209.1729 0 0

Tanglewood 91 614.48 2169085.083 25679.85251 0.011839025

Trib. No.6 126.4 527.72 16704028.7 127328.2486 0.007622607

Trib No. 5 55.4 452.7 10166940.75 82606.01135 0.008124962

Franklin 24.88 445.04 2746798.363 0 0

Troll 60.1 545.04 1622852.608 16539.27481 0.010191483

Trib No. 3 97.7 403.9 8388432.662 60518.8221 0.007214557

Lanier 26.12 436.06 13502549.45 37507.29507 0.002777794

Lower Ribelin 25.82 460.24 6868435.733 22985.12374 0.003346486

Stillhouse Hollow 128.9 573.48 453045.8908 190.02411 0.000419437

Barrow 98.3 493.56 1569744.12 12269.06727 0.007815966 Table 1

Figure 14

Figure 15

R² = 0.0354

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 20 40 60 80 100 120 140

Pip

e D

en

sity

Na+Cl

Pipe Density vs Na+Cl

R² = 0.3253

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 100 200 300 400 500 600 700

Pip

e D

en

sity

Ca+HCO3

Pipedensity vs Ca+HCO3

Figure 16

Figure 17

Figure 18

CONCLUSION

Assuming the following: H2O + CaCO3 + CO2 = Ca2+ + 2HCO3

- The dominant ions in natural water are Ca and HCO3 the addition of municipal water increases

concentration Na and Cl relative to Ca and HCO3.To prove my hypothesis a positive correlation should

be exist by plotting pipe densities vs chemical concentrations. The results obtained in this study do not

show any correlation between pipe density and chemical concentrations. In conclusion water pipe

density does not reflect an influence on the water chemistry of the springs tested.