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Exhibit 10.5

EXECUTION COPY

TETRA TECH, INC.

FIRST AMENDMENT TO
NOTE PURCHASE AGREEMENT

$110,000,000
Senior Secured Notes

$92,000,000
7.28% Senior Secured Notes,
Series A, due May 30, 2011

$18,000,000
7.08% Senior Secured Notes,
Series B, due May 30, 2008

Dated as of September 30, 2001

To the Holders of the Senior Notes
of Tetra Tech, Inc. Named in
the Attached Schedule I

Ladies and Gentlemen:

    Reference is made to the Note Purchase Agreement dated as of May 15, 2001
(the "Note Agreement") between Tetra Tech, Inc., a Delaware corporation (the
"Company"), and you pursuant to which the Company issued $92,000,000 aggregate
principal amount of its 7.28% Senior Secured Notes, Series A, due May 30, 2011
(the "Series A Notes") and $18,000,000 aggregate principal amount of its 7.08%
Senior Secured Notes, Series B, due May 30, 2008 (the "Series B Notes" and,
together with the Series A Notes, the "Notes"). You are referred to herein
individually as a "Holder" and collectively as the "Holders". Capitalized terms
used and not otherwise defined herein shall have the meanings ascribed to them
in the Note Agreement, as amended hereby.

    The Company has requested that the Note Agreement be amended to exclude a
special charge related to the bankruptcy of Metricom, Inc. from Adjusted EBITDA
for purposes of the Indebtedness covenant. You have agreed to such amendment on
the terms and conditions set forth herein.

    In consideration of the premises and for good and valuable consideration,
the receipt and sufficiency of which are acknowledged, the Company and the
Holders agree as follows:

1.  AMENDMENT OF NOTE AGREEMENT

    1.1. Section 10.1(a). Section 10.1(a) of the Note Agreement is amended to
read in its entirety as follows:

    "(a) the ratio of Consolidated Indebtedness (as of any date) to Adjusted
EBITDA (for the Company's then most recently completed four fiscal quarters) to
be greater that 2.75 to 1.00; provided, however, that for purposes of this
Section 10.1(a), Adjusted EBITDA shall be adjusted by (x) adding back the
special charge (but not more than $38,300,000) taken by the Company in the
fiscal quarter ended July 1, 2001 in connection with the bankruptcy of
Metricom, Inc. to the extent taken during the Company's then most recently
completed four fiscal quarters and (y) deducting any recovery received on the
obligations that gave rise to the special charge referred

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to in clause (x) to the extent received during the Company's then most recently
completed four fiscal quarters; or"

2.  REAFFIRMATION; REPRESENTATIONS AND WARRANTIES`

    2.1. Reaffirmation of Note Agreement. The Company reaffirms its agreement to
comply with each of the covenants, agreements and other provisions of the Note
Agreement and the Notes, including the amendment of such provisions effected by
this First Amendment.

    2.2. Note Agreement. The Company represents and warrants that the
representations and warranties contained in the Note Agreement are true and
correct as of the date hereof, except (a) to the extent that any of such
representations and warranties specifically relate to an earlier date, (b) for
such changes, facts, transactions and occurrences that have arisen since May 22,
2001 in the ordinary course of business, (c) for such other matters as have been
previously disclosed in writing by the Company (including in its financial
statements and notes thereto) to the Holders and (d) for other changes that
could not reasonably be expected to have a Material Adverse Effect.

    2.3. No Default or Event of Default. After giving effect to the transactions
contemplated hereby, there will exist no Default or Event of Default.

    2.4. Authorization. The execution, delivery and performance by the Company
of this First Amendment have been duly authorized by all necessary corporate
action and, except as provided herein, do not require any registration with,
consent or approval of, notice to or action by, any Person (including any
Governmental Authority) in order to be effective and enforceable. The Note
Agreement and this First Amendment each constitute the legal, valid and binding
obligations of the Company, enforceable in accordance with their respective
terms, except as such enforceability may be limited by (i) applicable
bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium or
other similar laws affecting the enforcement of creditors' rights generally and
(ii) general principles of equity (regardless of whether such enforceability is
considered in a proceeding in equity or at law).

3.  EFFECTIVE DATE

    This First Amendment shall become effective as of the date set forth above
upon the satisfaction of the following conditions:

    3.1. Consent of Holders to First Amendment. Execution by the Holders of at
least a majority of the aggregate principal amount of the Notes outstanding and
receipt by the Holders of a counterpart of this First Amendment duly executed by
the Company.

    3.2. Amendment Fee. Each Holder shall have received payment of an amendment
fee equal to 0.1% of the principal amount of the outstanding Notes held by such
Holder.

    3.3. Expenses. The Company shall have paid all fees and expenses of special
counsel to the Holders.

4.  MISCELLANEOUS

    4.1. Ratification. Except as amended hereby, the Note Agreement, including
the representations and warranties contained therein, shall remain in full force
and effect and is ratified, approved and confirmed in all respects as of the
date hereof.

    4.2. Reference to and Effect on the Note Agreement. Upon the final
effectiveness of this First Amendment, each reference in the Note Agreement and
in other documents describing or referencing the Note Agreement to the
"Agreement," "Note Agreement," "hereunder," "hereof," "herein," or

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words of like import referring to the Note Agreement, shall mean and be a
reference to the Note Agreement, as amended hereby.

    4.3. Binding Effect. This First Amendment shall be binding upon and inure to
the benefit of the respective successors and assigns of the parties hereto.

    4.4. Governing Law. This First Amendment shall be governed by and construed
in accordance with Illinois law.

    4.5. Counterparts. This First Amendment may be executed in any number of
counterparts, each executed counterpart constituting an original, but altogether
only one instrument.

    IN WITNESS WHEREOF, the Company and the Holders have caused this First
Amendment to be executed and delivered by their respective officer or officers
thereunto duly authorized.

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

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    Name:   James M. Jaska

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    Title:   Chief Financial Officer

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HOLDERS:

The foregoing is agreed
to as of the date thereof.

MASSMUTUAL ASIA LIMITED     By:   David L. Babson & Company Inc. as Investment
Adviser    
 
 
 
 
  By:   /s/ MARK A. AHMED   

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    Name:   Mark A. Ahmed

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    Title:   Managing Director

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  MASSACHUSETTS MUTUAL LIFE INSURANCE COMPANY     By:   David L. Babson &
Company Inc. as Investment Adviser    
 
 
 
 
  By:   /s/ MARK A. AHMED   

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    Name:   Mark A. Ahmed

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    Title:   Managing Director

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  C.M. LIFE INSURANCE COMPANY     By:   David L. Babson & Company Inc. as
Investment Adviser    
 
 
 
 
  By:   /s/ MARK A. AHMED   

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    Name:   Mark A. Ahmed

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    Title:   Managing Director

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  CONNECTICUT GENERAL LIFE INSURANCE COMPANY     By:   CIGNA Investments, Inc.
(authorized agent)    
 
 
 
 
  By:   /s/ STEPHEN H. WILSON   

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    Name:   Stephen H. Wilson

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    Title:   Managing Director

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  LIFE INSURANCE COMPANY OF NORTH AMERICA     By:   CIGNA Investments, Inc.
(authorized agent)    
 
 
 
 
  By:   /s/ STEPHEN H. WILSON   

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    Name:   Stephen H. Wilson

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    Title:   Managing Director

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  UNITED OF OMAHA LIFE INSURANCE COMPANY    
 
 
 
 
  By:   /s/ EDWIN H. GARRISON JR.   

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    Name:   Edwin H. Garrison Jr.

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    Title:   First Vice President

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  MUTUAL OF OMAHA INSURANCE COMPANY    
 
 
 
 
  By:   /s/ EDWIN H. GARRISON JR.   

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    Name:   Edwin H. Garrison Jr.

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    Title:   First Vice President

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  HARTFORD LIFE INSURANCE COMPANY     By:   Hartford Investment Services, Inc.,
its Agent and Attorney-in-Fact    
 
 
 
 
  By:   /s/ KURT H. NYMAN   

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    Name:   Kurt H. Nyman

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    Title:   Vice President

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  NATIONWIDE LIFE INSURANCE COMPANY    
 
 
 
 
  By:   /s/ MARK W. POEPPELMAN   

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    Name:   Mark W. Poeppelman

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    Title:   Associate Vice President

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  NATIONWIDE LIFE AND ANNUITY INSURANCE COMPANY    
 
 
 
 
  By:   /s/ MARK W. POEPPELMAN   

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    Name:   Mark W. Poeppelman

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    Title:   Associate Vice President

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  PROVIDENT MUTUAL LIFE INSURANCE COMPANY    
 
 
 
 
  By:   /s/ JAMES D. KESTNER   

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    Name:   James D. Kestner

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    Title:   Vice President

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  SECURITY FINANCIAL LIFE INSURANCE CO.    
 
 
 
 
  By:   /s/ KEVIN W. HAMMOND   

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    Name:   Kevin W. Hammond

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    Title:   Chief Investment Officer

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  THE CANADA LIFE ASSURANCE COMPANY    
 
 
 
 
  By:   /s/ PAUL ENGLISH   

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    Name:   Paul English

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    Title:   Associate Treasurer

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  CANADA LIFE ASSURANCE COMPANY OF NEW YORK    
 
 
 
 
  By:   /s/ PAUL ENGLISH   

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    Name:   Paul English

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    Title:   Associate Treasurer

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  LUTHERAN BROTHERHOOD    
 
 
 
 
  By:   /s/ KERI L. REICH   

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    Name:   Keri L. Reich

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    Title:   Portfolio Manager

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  MODERN WOODMEN OF AMERICA    
 
 
 
 
  By:   /s/ NICK S. COIN   

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    Name:   Nick S. Coin

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    Title:   Manager, Securities Division

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SCHEDULE I

Series A Holders

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  Principal
Amount

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Massachusetts Mutual Life Insurance Company   $ 21,500,000 C.M. Life Insurance
Company     3,000,000 MassMutual Asia Limited     500,000 Connecticut General
Life Insurance Company     17,000,000 Life Insurance Company of North America  
  3,000,000 United of Omaha Life Insurance Company     12,000,000 Mutual of
Omaha Insurance Company     3,000,000 Hartford Life Insurance Company    
15,000,000 Nationwide Life Insurance Company     7,000,000 Nationwide Life and
Annuity Insurance Company     3,000,000 Provident Mutual Life Insurance Company
    5,000,000 Security Financial Life Insurance Co.     2,000,000

Series B Holders

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  Principal
Amount

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The Canada Life Assurance Company   $ 8,700,000 Canada Life Insurance Company of
New York     300,000 Lutheran Brotherhood     6,000,000 Modern Woodmen of
America     3,000,000

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QuickLinks

Exhibit 10.5 EXECUTION COPY
TETRA TECH, INC. FIRST AMENDMENT TO NOTE PURCHASE AGREEMENT $110,000,000 Senior
Secured Notes $92,000,000 7.28% Senior Secured Notes, Series A, due May 30, 2011
$18,000,000 7.08% Senior Secured Notes, Series B, due May 30, 2008
Dated as of September 30, 2001
SCHEDULE I