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Exhibit 10.3

SECOND AMENDMENT

    THIS SECOND AMENDMENT (this "Amendment") dated as of August 13, 2001 is to
the Credit Agreement (the "Credit Agreement") dated as of March 17, 2000 among
TETRA TECH, INC. (the "Company"), various financial institutions and BANK OF
AMERICA, N.A., as administrative agent (the "Agent"). Unless otherwise defined
herein, terms defined in the Credit Agreement are used herein as defined
therein.

    WHEREAS, the parties hereto have entered into the Credit Agreement which
provides for the Banks to make Loans to, and to issue Letters of Credit for the
account of, the Company from time to time; and

    WHEREAS, the parties hereto desire to amend the Credit Agreement as set
forth below;

    NOW THEREFORE, in consideration of the premises and for other good and
valuable consideration (the receipt and sufficiency of which are hereby
acknowledged), the parties hereto agree as follows:

    SECTION 1 AMENDMENTS. Effective on (and subject to the occurrence of) the
Amendment Effective Date (as defined below), the Credit Agreement shall be
amended as follows:

    Section 1.1 Definitions. Section 1 of the Credit Agreement is amended by
inserting the following definition in the appropriate alphabetical order:

    "Adjusted Leverage Ratio means, for any Computation Period, the Leverage
Ratio for such Computation Period, provided that for purposes of computing the
Adjusted Leverage Ratio, EBITDA shall be adjusted by (x) adding back the special
charge (but not more than $38,300,000) taken by the Company in the third Fiscal
Quarter of Fiscal Year 2001 in connection with the bankruptcy of Metricom, Inc.,
to the extent taken in such Computation Period; and (y) deducting any recovery
received on the obligations which gave rise to the special charge referred to in
clause (x), to the extent received during such Computation Period."

    Section 1.2 Leverage Covenant. Section 10.6.3 of the Credit Agreement is
amended by inserting the word "Adjusted" before the words "Leverage Ratio"
therein.

    Section 1.3 Addition of Covenant. Section 10 of the Credit Agreement is
amended by adding the following:

    "10.23 Note Purchase Agreement Covenants. Comply with the terms set forth in
Sections 10.1 (Indebtedness; Priority Debt), 10.2 (Fixed Charge Coverage) and
10.3 (Adjusted Consolidated Net Worth) of the Note Purchase Agreement dated as
of May 15, 2001 executed in connection with the Private Placement Debt, as such
Note Purchase Agreement is in effect on the date hereof (without giving effect
to any amendment thereto or waiver thereof unless consented to by the Required
Banks)."

    Section 1.4 Events of Default. Clause (a) of Section 12.1.5 of the Credit
Agreement is amended by adding the following at the end thereof (after the
reference to "10.21" and before the semi-colon): "and 10.23".

    SECTION 2 REPRESENTATIONS AND WARRANTIES. The Company represents and
warrants to the Agent and the Banks that (a) each warranty set forth in
Section 9 of the Credit Agreement is true and correct as if made on the date
hereof, (b) the execution and delivery by the Company of this Amendment and the
performance by the Company of its obligations under the Credit Agreement as
amended hereby (as so amended, the "Amended Credit Agreement") (i) are within
the corporate powers of the Company, (ii) have been duly authorized by all
necessary corporate action, (iii) have received all

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necessary governmental approval and (iv) do not and will not contravene or
conflict with any provision of law or of the charter or by-laws of the Company
or any Subsidiary or of any indenture, loan agreement or other material
contract, or any judgment, order or decree, which is binding upon the Company or
any Subsidiary, and (c) this Amendment and the Amended Credit Agreement are the
legal, valid and binding obligations of the Company, enforceable against the
Company in accordance with their terms, except as enforceability may be limited
by bankruptcy, insolvency or other similar laws of general application affecting
the enforcement of creditor's rights or by general principles of equity.

    SECTION 3 EFFECTIVENESS. The amendments set forth in Section 1 shall become
effective, as of the day and year first above written, on such date (the
"Amendment Effective Date") that the Agent shall have received each of the
following: (a) counterparts of this Amendment executed by the Company and the
Required Banks; (b) a Confirmation in the form of Exhibit A hereto signed by the
Company and each Subsidiary; (c) for the account of each Bank that has executed
and delivered a counterpart hereof to the agent (via facsimile or otherwise) by
1:00 PM (Chicago Time) on August 13, 2001, an amendment fee in an amount equal
to 0.10% of such Bank's commitment; and (d) such other documents as the Agent or
any Bank may reasonably request.

    SECTION 4 MISCELLANEOUS.

    Section 4.1 Continuing Effectiveness, etc. As herein amended, the Credit
Agreement shall remain in full force and effect and is hereby ratified and
confirmed in all respects.

    Section 4.2 Counterparts. This Amendment may be executed in any number of
counterparts and by the different parties on separate counterparts, and each
such counterpart shall be deemed to be an original but all such counterparts
shall together constitute one and the same Second Amendment.

    Section 4.3 Governing Law. This Amendment shall be a contract made under and
governed by the laws of the State of Illinois applicable to contracts made and
to be performed entirely within such State.

    Section 4.4 Successors and Assigns. This Amendment shall be binding upon the
Company and the Agent and their respective successors and assigns, and shall
inure to the benefit of the Company and the Agent and the successors and assigns
of the Agent.

    Section 4.5 Expenses. The Company agrees to pay the reasonable costs and
expenses of the Agent (including attorneys' fees) in connection with the
preparation, execution and delivery of this Amendment.

    Delivered at Chicago, Illinois, as of the day and year first above written.

    TETRA TECH, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Chief Financial Officer

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      BANK OF AMERICA, N.A., AS
ADMINISTRATIVE AGENT
 
 
 
 
      By:   /s/ KRISTINE D HYDE   

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    Title:   Vice President

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      BANK OF AMERICA, N.A., AS SWING LINE
BANK, ISSUING BANK AND AS A BANK
 
 
 
 
      By:   /s/ JENNIFER L. GERDES   

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    Title:   Vice President

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      WELLS FARGO BANK, N.A., AS
DOCUMENTATION AGENT AND AS A BANK
 
 
 
 
      By:   /s/ RANDALL J. REPP   

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    Title:   Vice President

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      HARRIS TRUST AND SAVINGS BANK,
AS SYNDICATION AGENT AND AS A BANK
 
 
 
 
      By:   /s/ ISABELLA BATTISTA   

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    Title:   Vice President

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      THE FUJI BANK, LIMITED
 
 
 
 
      By:   /s/ STEVEN T. BRENNAN   

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    Title:   Senior Vice President

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      PACIFIC CENTURY BANK, N.A.
 
 
 
 
      By:  

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    Title:  

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EXHIBIT A

CONFIRMATION

Dated as of August 13, 2001

    Each of the undersigned hereby acknowledges and agrees to the foregoing
Second Amendment and the Amended Credit Agreement and hereby confirms the
continuing validity and enforceability of the Guaranty and the Security
Agreement after giving effect thereto.

    HSI GEOTRANS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Assistant Treasurer

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      SCM CONSULTANTS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Assistant Treasurer

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      TETRA TECH EM, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      WHALEN & COMPANY, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Chief Financial Officer

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      TETRA TECH NUS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      MFG, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      COLLINS-PINA CONSULTING ENGINEERS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      DEA CONSTRUCTION COMPANY, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      BAHA COMMUNICATIONS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      UTILITIES & C.C., INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      ASL CONSULTING ENGINEERS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      ASL CONSULTING ENGINEERS ARIZONA, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      ASL CONSULTANTS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      COSENTINI ASSOCIATES, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      EVERGREEN UTILITY CONTRACTORS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      KCM, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Assistant Treasurer

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      LC OF ILLINOIS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Treasurer

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      MCNAMEE, PORTER & SEELEY, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Assistant Treasurer

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      MCNAMEE INDUSTRIAL SERVICES, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Assistant Treasurer

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      PDR ENGINEERS, INC.
 
 
 
 
      By:   /s/ JAMES M. JASKA   

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    Title:   Chief Financial Officer

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Exhibit 10.3
SECOND AMENDMENT
EXHIBIT A CONFIRMATION Dated as of August 13, 2001