EXHIBIT 4

                           SHARE REPURCHASE AGREEMENT


          SHARE REPURCHASE AGREEMENT (this "Agreement"), dated as of January 15,
1998, among OHM Corporation, an Ohio corporation (the "Company"), Waste
Management, Inc., a Delaware corporation (the "Shareholder"), Rust International
Inc., a Delaware corporation ("Rust"), and International Technology Corporation,
a Delaware corporation ("Parent").

          WHEREAS, certain of the parties to this Agreement are also parties to
the Standstill and Non-Competition Agreement, dated as of May 30, 1995, among
the Company, the Shareholder and Rust (the "Standstill Agreement"); and

          WHEREAS, concurrently with the execution of this Agreement, the
Company, Parent and IT-Ohio, Inc., an Ohio corporation ("Merger Sub"), have
entered into an Agreement and Plan of Merger, dated as of the date hereof (as it
may be amended from time to time, the "Merger Agreement"), which provides that
Merger Sub will make a tender offer (the "Offer") for 13,933,000 shares of
Common Stock, par value $0.10 per share, of the Company ("Shares") and that,
subsequent to the consummation of the Offer, Merger Sub will merge with and into
the Company (the "Merger" and, collectively with the Offer and the other
transactions contemplated by the Merger Agreement, the "Merger Transactions");
and

          WHEREAS, the Shareholder is the record holder of an aggregate of
9,668,000 Shares (the "Shareholder Shares"); and

          WHEREAS, Parent, the Company and the Shareholder wish, as a part of
the Merger Transactions, to provide for the repurchase by the Company from the
Shareholder, concurrently with the payment to BankBoston, N.A., as Depositary
for the Offer on behalf of holders of Shares tendering into the Offer, of the
aggregate purchase price for all Shares purchased in the Offer (the "Payment
Time"), of 5,235,381 Shareholder Shares (the "Repurchased Shares"), in a manner
that will increase the aggregate number of Shares acquired for cash in the
Merger Transactions and make it possible for the Merger Consideration (as
defined in the Merger Agreement) to consist solely of shares of Parent Common
Stock (as defined in the Merger Agreement); and

 
          WHEREAS, the parties intend for the Shareholder not to receive any
greater consideration per Share in the Merger Transactions than the other
holders of Shares, and that for Shareholder not to receive any greater amount of
cash consideration per Share in the Merger Transactions than the other holders
of Shares; and

          WHEREAS, in order to facilitate consummation of the Merger
Transactions, the Shareholder wishes to agree (i) to vote the Shareholder Shares
and any other shares of capital stock of the Company held by it so as to
facilitate consummation of the Merger Transactions, (ii) except as provided in
this Agreement, not to transfer or otherwise dispose of any of the Shareholder
Shares, or any other shares of capital stock of the Company, acquired by it
hereafter and prior to the Effective Time (as defined in the Merger Agreement),
(iii) to deliver to Parent an irrevocable proxy to vote the Shareholder Shares
and any other shares of capital stock of the Company acquired by the Shareholder
hereafter and prior to the Effective Time, and (iv) to amend or terminate, as
the case may be, certain agreements to which certain of the parties hereto are
parties, and (v) to make certain other agreements, all as provided for herein.

          NOW, THEREFORE, for good and valuable considera  tion, the receipt,
sufficiency and adequacy of which is hereby acknowledged, the parties hereto
agree as follows:


                                   ARTICLE I

                  Definitions; Representations and Warranties
                  -------------------------------------------

          1.1  Definitions.  Terms used herein but not defined herein shall have
               -----------                                                      
the respective meanings ascribed to such terms in the Merger Agreement.

          1.2  Representations of the Shareholder.  The Shareholder represents
               ----------------------------------                             
and warrants to the Company that (a) it owns beneficially (as such term is
defined in the Securities Exchange Act of 1934, as amended (the "1934 Act"))
9,668,000 Shares free and clear of all liens, claims, charges, security
interests or other encumbrances (each, a "Lien") and, except for this Agreement
and the warrants to purchase Shares (the "Warrants") issued pursuant to the
Warrant Agreement, dated as of May 30, 1995, among the Company and the
Shareholder (the "Warrant Agreement"), there are no options, warrants or other
rights, agreements, 

                                      -2-

 
arrangements or commitments of any character to which the Shareholder is a party
relating to the pledge or disposition of any shares of capital stock of the
Company and, except for the Standstill Agreement and this Agreement, there are
no voting trusts or voting agreements to which the Shareholder is a party with
respect to any shares of capital stock of the Company; (b) the Shareholder does
not beneficially own any shares of capital stock of the Company other than the
Shareholder Shares and the Warrants and, except for the Warrants, does not have
any options, warrants or other rights to acquire any additional shares of
capital stock of the Company or any security exercisable for or convertible into
shares of capital stock of the Company; (c) the Shareholder has full power and
authority to enter into, execute and deliver this Agreement and to perform fully
its obligations under this Agreement; and (d) this Agreement has been duly
executed and delivered by each of the Shareholder and Rust, constitutes the
legal, valid and binding obligation of the Shareholder and Rust and is
enforceable against each of them in accordance with its terms. The foregoing
representations shall survive consummation of the Merger Transactions and the
other transactions contemplated by this Agreement.

          1.3  Representations of the Company.  The Company represents and
               ------------------------------                             
warrants to the Shareholder that (a) the Company has full power and authority to
enter into, execute and deliver this Agreement and to perform fully its
obligations under this Agreement, (b) this Agreement has been duly executed and
delivered by the Company, constitutes the legal, valid and binding obligation of
the Company and is enforceable against it in accordance with its terms, and (c)
the Company has obtained all consents, approvals, permits and authorizations
required to be obtained by the Company pursuant to any law, regulation,
contract, agreement or instrument in connection with the execution and delivery
of this Agreement.  The foregoing representations shall survive consummation of
the Merger Transactions and the other transactions contemplated by this
Agreement.


                                   ARTICLE II

                          The Repurchase and the Offer
                          ----------------------------

          2.1  Repurchase of Shares.  (a)  Subject to the terms and conditions
               --------------------                                           
of this Agreement, including the conditions set forth in Section 2.3, the
Company agrees to 

                                      -3-

 
purchase from the Shareholder, and the Shareholder agrees to sell to the Company
(such purchase and sale transaction, the "Repurchase"), the Repurchased Shares,
free and clear of any Liens at a purchase price of $11.50 per Repurchased Share,
or such greater price per Repurchased Share as may be paid in the Offer (the
"Repurchase Price").

          (b) If for any reason the Company has not repurchased the Repurchased
Shares immediately prior to the Effective Time (as defined in the Merger
Agreement), the Company shall take such action as may be necessary at such time
to purchase the Repurchased Shares at the Repurchase Price for cash at such
time, so that the Repurchased Shares shall have been acquired by the Company,
Parent or Merger Sub prior to the Effective Time.

          2.2  Repurchase Closing.  (a) The delivery of the Repurchased Shares
               ------------------                                             
(the "Repurchase Closing") shall take place at the offices of Sullivan &
Cromwell, 125 Broad Street, New York, New York at the Payment Time.

          (b)  At the Repurchase Closing:

               (i) The Shareholder shall deliver to the Company certificates
     representing the Repurchased Shares, duly endorsed and in form for transfer
     to the Company; and

               (ii) The Company shall pay to the Shareholder, by wire transfer,
     to an account designated by the Shareholder no fewer than two business days
     prior to the Repurchase Closing, immediately available funds equivalent to
     the Repurchase Price multiplied by the number of Repurchased Shares.

          2.3  Conditions to the Repurchase.  (a)  The respective obligations of
               ----------------------------                                     
the Company and the Shareholder to consummate the Repurchase are subject to the
fulfillment of each of the following conditions, any or all of which may be
waived in whole or in part by the Company or the Shareholder, as the case may
be, to the extent permitted by applicable law:

          (i) Concurrently with the Repurchase Closing, Merger Sub shall have
     paid for Shares pursuant to the Offer.

                                      -4-

 
         (ii) No United States or state court or other Governmental Entity (as
     defined in the Merger Agreement) of competent jurisdiction shall have
     enacted, issued, promulgated, enforced or entered any statute, rule,
     regulation, judgment, decree, injunction or other order (whether temporary,
     preliminary or permanent) which is in effect and prohibits consummation of
     the transactions contemplated by the Merger Agreement or this Agreement.

          (b) The obligation of the Company to consummate the Repurchase is
subject to the condition that the representations and warranties of the
Shareholder contained in Section 1.2 are true and accurate in all material
respects as of the date hereof and as of the Repurchase Closing, provided,
however, that the foregoing condition may be waived in whole or in part by the
Company.

          (c) The obligation of the Shareholder to consummate the Repurchase is
subject to the condition that the representations and warranties of the Company
contained in Section 1.3 are true and accurate in all material respects as of
the date hereof and as of the Repurchase Closing, provided, however, that the
foregoing condition may be waived in whole or in part by the Shareholder.

          2.4  The Tender Offer.  The Shareholder agrees that it will not tender
               ----------------                                                 
more than 2,142,141 Shareholder Shares into the Offer.


                                  ARTICLE III

                                   The Merger
                                   ----------

          3.1  Agreement to Vote Shares.  In addition to and notwithstanding the
               ------------------------                                         
provisions of Section 1.2 of the Standstill Agreement, the Shareholder agrees
that during the term of this Agreement it consents to and approves the voting of
the Shareholder Shares and any New Shares (as defined in Section 4.2), (a) in
favor of adoption of the Merger Agreement and in favor of consummation of the
Merger Transactions at every meeting of the shareholders of the Company at which
such matters are considered and at every adjournment thereof or in connection
with any written consent of the shareholders of the Company, (b) in favor of the
election to the Company's Board of Directors of such 

                                      -5-

 
number of Parent Representatives as Parent is permitted to cause to be elected
to the Company's Board of Directors pursuant to Section 1.4 of the Merger
Agreement, (c) against any action or agreement that would compete with, impede,
interfere with or attempt to discourage the Merger Transactions, or inhibit the
timely consummation of the Merger Transactions, (d) against any action or
agreement that would result in a breach in any material respect of any covenant,
representation or warranty or any other obligation of the Company under the
Merger Agreement and (e) against any merger, consolidation, business
combination, reorganization, recapitalization, liquidation or sale or transfer
of any material assets of the Company or its subsidiaries, except for the Merger
Transactions. The Shareholder agrees to deliver to Parent upon request a proxy
substantially in the form attached hereto as Exhibit A, which proxy shall be
irrevocable during the term of this Agreement to the fullest extent permitted
under Ohio law.

          3.2  No Voting Trusts.  The Shareholder agrees that it will not, nor
               ----------------                                               
will it permit any entity under their control to, deposit any of the Shareholder
Shares or any New Shares held by it in a voting trust or subject any of the
Shareholder Shares or any New Shares held by it to any arrangement with respect
to the voting of the Shareholder Shares that could result in a shareholder's
vote or action by consent of the shareholders of the Company in opposition to or
in competition with the consummation of the Merger Transactions.

          3.3  No Proxy Solicitations.  The Shareholder agrees that it will not,
               ----------------------                                           
nor will it permit any entity under its control to, (a) solicit proxies or
become a "participant" in a "solicitation" (as such terms are defined in
Regulation 14A under the 1934 Act) in opposition to or competition with the
consummation of the Merger Transactions or otherwise encourage or assist any
party in taking or planning any action which would compete with, impede,
interfere with or attempt to discourage the Merger Transactions or inhibit the
timely consummation of the Merger Transactions, (b) directly or indirectly
encourage, initiate or cooperate in a shareholders' vote or action by consent of
the Company's shareholders in opposition to or in competition with the
consummation of the Merger Transactions, or (c) become a member of a "group" (as
such term is used in Section 13(d) of the 1934 Act) with respect to any voting
securities of the Company for the purpose of 

                                      -6-

 
opposing or competing with the consummation of the Merger Transactions.

          3.4  Waiver of Dissenters' Rights.  The Shareholder hereby
               ----------------------------                         
unconditionally and irrevocably waives its rights pursuant to Sections 1701.84
                                                                              
et seq. of the Ohio General Corporation Law to exercise appraisal rights or
- -- ---                                                                     
dissenters' rights with respect to the Offer, the Merger, or the other
transactions contemplated by the Merger Agreement.


                                   ARTICLE IV

                                Other Agreements
                                ----------------

          4.1  No Transfer or Encumbrance.  In addition to and notwithstanding
               --------------------------                                     
the provisions of Section 1.8 of the Standstill Agreement, the Shareholder
agrees not to transfer, sell, offer, exchange, pledge or otherwise dispose of or
encumber any of the Warrants, Shareholder Shares or New Shares on or after the
date hereof and during the term of this Agreement, except for tenders in
accordance with Section 2.4, unless the transferee agrees in writing in form
satisfactory to the Company and Parent to be bound by the terms of this
Agreement.

          4.2  No Additional Purchases or Acquisitions.  In addition to and
               ---------------------------------------                     
notwithstanding the provisions of Sections 1.1 and 2.4 of the Standstill
Agreement, the Shareholder agrees that it will not purchase or otherwise acquire
beneficial ownership of any Shares or any other capital stock of the Company
after the execution of this Agreement ("New Shares"), nor will the Shareholder
voluntarily acquire the right to vote or share in the voting of any Shares or
any other capital stock of the Company other than the Shareholder Shares, unless
(in either case) the Shareholder agrees to deliver to the Board of Directors of
the Company immediately after such purchase or acquisition an irrevocable proxy
in the form attached hereto as Exhibit A with respect to such New Shares.  The
Shareholder also agrees that any New Shares acquired or purchased by it shall be
subject to the terms of this Agreement to the same extent as if they constituted
Shareholder Shares.

          4.3  First Amendment to Standstill Agreement.  The Standstill
               ---------------------------------------                 
Agreement is hereby amended ab initio, as of the 

                                      -7-

 
execution of this Agreement, to amend Section 1.4 thereof by adding the
following clause to the end of such Section:

     ", except the Share Repurchase Agreement, dated January 15, 1998, among OHM
     Corporation, Waste Management, Inc., Rust International Inc. and
     International Technology Corporation."

          4.4  Second Amendment to Standstill Agreement. The Standstill
               ----------------------------------------                
Agreement is hereby further amended ab initio, as of the occurrence of the
Repurchase Closing, to delete therefrom Sections 2.1 through 2.7 thereof in
their entirety.

          4.5  Third Amendment to Standstill Agreement.  The Standstill
               ---------------------------------------                 
Agreement is hereby further amended ab initio, as of the Repurchase Closing, to
delete therefrom Sections 3.1, 3.2 and 3.3 thereof in their entirety.

          4.6  Release from Intercreditor Agreement.  The Shareholder hereby
               ------------------------------------                         
consents to the payment by the Company of a pro rata taxable distribution (the
                                            --- ----                          
"NSC Distribution") to holders of record of the Shares of all of the shares of
common stock, par value $0.01 per share, of NSC Corporation held by the Company
(the "NSC Shares") and waives its rights to reimbursement pursuant to the
Reimbursement Agreement, dated as of May 31, 1995, among the Company,
Remediation and the Shareholder, and the Intercreditor Agreement, dated as of
May 31, 1995, among the Shareholder, the Administrative Agent and the Issuing
and Paying Agent, and hereby releases the NSC Shares from any security interest
(pursuant to pledge agreements or otherwise) which the Shareholder may have with
respect to such NSC Shares.  The Shareholder agrees to execute any documents
reasonably necessary to give effect to the provisions of this Section, promptly
upon request therefor made by the Company.

          4.7  Termination of the Guaranty Agreement.  The parties hereby agree
               -------------------------------------                           
that the Guaranty Agreement, dated as of May 30, 1995, between the Company and
the Shareholder, shall be terminated effective as of the Common Termination
Date.

          4.8  Cancellation of The Warrants.
               ---------------------------- 

          (a)  The parties hereby agree that the Warrant Agreement shall be
terminated and the Warrants shall be 

                                      -8-

 
canceled effective as of the date on which the Guaranty, made as of May 31,
1995, by the Shareholder in favor of the Banks listed therein (the "Guaranty")
terminates in accordance with Section 10(a) thereof (such date, the "Common
Termination Date"), without the payment of any separate consideration therefor.
The parties hereby agree to use their reasonable best efforts to cause the
events specified in Section 10(a) of the Guaranty to occur prior to the second
business day subsequent to the Effective Time.

          (b) The Shareholder agrees not to exercise its rights pursuant to
Sections 3.1 or 3.2 of the Warrant Agreement prior to the earlier to occur of
(i) the second business day subsequent to the Effective Time, and (ii) the
termination of the Merger Agreement in accordance with its terms.

          (c) The Shareholder agrees not to exercise its rights pursuant to
Section 2.2 of the Warrant Agreement with respect to the Merger Transactions
prior to the second business day subsequent to the Effective Time.

          (d) The Shareholder hereby waives its rights under Sections 2.1
through 2.7 of the Warrant Agreement with respect to the NSC Distribution.


                                   ARTICLE V

                                 Miscellaneous
                                 -------------

          5.1  Specific Performance.  Each party hereto acknowledges that it
               --------------------                                         
will be impossible to measure in money the damage to the other party if a party
hereto fails to comply with any of the obligations imposed by this Agreement,
that every such obligation is material and that, in the event of any such
failure, the other party will not have an adequate remedy at law or damages.
Accordingly, each party hereto agrees that injunctive relief or other equitable
remedy, in addition to remedies at law or damages, is the appropriate remedy for
any such failure and will not oppose the granting of such relief on the basis
that the other party has an adequate remedy at law.  Each party hereto agrees
that it will not seek, and agrees to waive any requirement for, the securing or
posting of a bond in connection with any other party's seeking or obtaining such
equitable relief.

                                      -9-

 
          5.2  Entire Agreement.  This Agreement and the Standstill Agreement
               ----------------                                              
(as herein amended) supersedes all prior agreements, written or oral, among the
parties hereto with respect to the subject matter hereof and contains the entire
agreement among the parties with respect to the subject matter hereof.  This
Agreement may not be amended, supplemented or modified, and no provisions hereof
may be modified or waived, except by an instrument in writing signed by all the
parties hereto.  No waiver of any provisions hereof by any party shall be deemed
a waiver of any other provisions hereof by any such party, nor shall any such
waiver be deemed a continuing waiver of any provision hereof by such party.

          5.3  Notices.  Any notice, request, instruction or other document to
               -------                                                        
be given hereunder by any party to the others shall be in writing and delivered
personally or sent by registered or certified mail, postage prepaid, or by
facsimile:

          if to the Shareholder or Rust:
          ----------------------------- 

          Herbert A. Getz
          Waste Management, Inc.
          3003 Butterfield Road
          Oak Brook, Illinois  60523
          Telecopier:  (630) 572-9130

          with a copy to:

          John H. Bitner
          Bell, Boyd & Lloyd
          Three First National Plaza
          70 West Madison Street, Suite 3300
          Chicago, Illinois  60602-4207
          Telecopier:  (312) 372-2098

          if to the Company:
          ----------------- 

          Steven E. Harbour
          OHM Corporation
          5445 Triangle Parkway, Suite 400
          Norcross, Georgia  30092
          Telecopier:  (770) 849-3110

                                      -10-

 
          with a copy to:

          Joseph B. Frumkin
          Sullivan & Cromwell
          125 Broad Street
          New York, New York 10004
          Telecopier:  (212) 558-3588

          and a copy to:

          Thomas C. Daniels
          Jones Day Reavis & Pogue
          North Point
          901 Lakeside Avenue
          Cleveland, Ohio 44114
          Telecopier:  (216) 579-0212

          if to Parent:
          ------------ 

          James G. Kirk
          International Technology Corporation
          2790 Mosside Boulevard
          Monroeville, PA  15146-2792
          Telecopier:  (412) 858-3978

          with a copy to:

          Peter F. Ziegler
          Gibson, Dunn & Crutcher LLP
          333 South Grand Avenue
          Los Angeles, California 90071
          Telecopier:  (213) 229-7520

or to such other persons or addresses as may be designated in writing by the
party to receive such notice as provided above.

          5.4  Miscellaneous.
               ------------- 

          (a) This Agreement shall be deemed a contract made under, and for all
purposes shall be construed in accordance with, the laws of the State of Ohio
applicable to agreements executed in and solely to be performed within such
State.

          (b)  If any provision of this Agreement or the application of such
provision to any person or circumstances shall be held invalid or unenforceable
by a court of 

                                      -11-

 
competent jurisdiction, such provision or application shall be unenforceable
only to the extent of such invalidity or unenforceability and the remainder of
the provision held invalid or unenforceable and the application of such
provision to persons or circumstances, other than the party as to which it is
held invalid, and the remainder of this Agreement, shall not be affected.

          (c)  This Agreement may be executed in one or more counterparts, each
of which shall be deemed to be an original but all of which together shall
constitute one and the same instrument.

          (d)  This Agreement shall terminate automatically upon the termination
of the Merger Agreement.  This Agreement may be terminated by the Company at any
time if the Shareholder shall have failed to comply with any of its covenants or
agreements contained in this Agreement.  This Agreement may be terminated by the
Shareholder at any time if the Company shall have failed to comply with any of
its covenants or agreements contained in this Agreement.

          (e)  Each party hereto shall execute and deliver such additional
documents as may be necessary or desirable to effect the transactions
contemplated by this Agreement.

          (f)  All Section headings herein are for convenience of reference only
and are not part of this Agreement, and no construction or reference shall be
derived therefrom.

          (g) The obligations of the parties set forth in this Agreement shall
not be effective or binding upon any party hereto until after such time as the
Merger Agreement is executed and delivered by the Company, Parent and Merger
Sub, and the parties agree that there is not and has not been any other
agreement, arrangement or understanding between the parties hereto with respect
to the matters set forth herein.

                                      -12-

 
          IN WITNESS WHEREOF, the parties hereto have executed and delivered
this Agreement as of the date first written above.

                              OHM CORPORATION



                              By:__________________________
                                 Name:
                                 Title:


                              WASTE MANAGEMENT, INC.



                              By:__________________________
                                 Name:
                                 Title:


                              RUST INTERNATIONAL INC.



                              By:__________________________
                                 Name:
                                 Title:


                              INTERNATIONAL TECHNOLOGY
                                CORPORATION



                              By:__________________________
                                 Name:
                                 Title:

 
                                                                     (EXHIBIT A)



                                 FORM OF PROXY

          The undersigned, for consideration received, hereby appoints Anthony
J. DeLuca and James G. Kirk and each of them my proxies, with power of
substitution and resubstitution, to vote all shares of Common Stock of OHM
Corporation, an Ohio corporation (the "Company"), [and [insert any other Shares
(as defined in the Share Repurchase Agreement) or other shares of capital stock
of the Company owned by the Shareholder)]] owned by the undersigned at the
Special Meeting of Shareholders of the Company to be held [insert date, time and
place] and at any adjournment thereof IN FAVOR OF adoption of the Agreement and
Plan of Merger, dated as of January 15, 1998 (the "Merger Agreement"), among the
Company, International Technology Corporation ("Parent") and IT-Ohio, Inc., IN
FAVOR OF consummation of the Merger Transactions, IN FAVOR OF [List such number
of Parent Representatives (as defined in Section 1.4 of the Merger Agreement) as
Parent is permitted to cause to be elected to the Company's Board of Directors
pursuant to Section 1.4 of the Merger Agreement], and AGAINST [insert
description of any action or agreement that would compete with, impede,
interfere with or attempt to discourage the Merger Transactions or inhibit the
timely consummation of the Merger Transactions or any action or agreement that
would result in a breach in any material respect of any covenant, representation
or warranty or any other obligation of the Company under the Merger Agreement or
any merger, consolidation, business combination, reorganization,
recapitalization, liquidation or sale or transfer of any material assets of the
Company or its subsidiaries].  This proxy is coupled with an interest, revokes
all prior proxies granted by the undersigned and is irrevocable until such time
as the Share Repurchase Agreement, dated as of January 15, 1998 among the
undersigned and the Company terminates in accordance with its terms.

                                               Dated _________, 1998
                                            
                                               RUST INTERNATIONAL, INC.
                                            
                                            
                                               By:___________________________
                                                  Name:
                                                  Title: